Temperature & pressure relieve valve leaking

The valves are the same specs. Is it possible that a new valve leaks, yew. However if both valve leak then there are other problems. Does the valve only leak when hot water is called for? Then check temperature setting on heater. Does it lek all the time? The water pressure is to high ,

• Maximum Temperature Setting (Deg. F): 210
• Maximum Pressure Setting (psi): 150

Why the relief valve at the water heater is leaking and what to do about it

Does tankless water heater require pressure relief valve

The T&P valve or temperature and pressure relief valve is placed on a hot water heater for safety. If it is still leaking after replacement either the temperature is too high causing excessive pressure or your house pressure regulator is set to high or has failed. The water heater has a thermostat to control the temperature. Check to make sure it isnt cranked all the way up. If the water heater is old there could be a lot of sediment build up. if sediment builds up near the temp sensor it will have to superheat the water above normal levels to heat the sediment up enough to shut off. Drain your water heater and flush out all the junk then try it again.

it could be a leak around an element if it is an electric heater.

Do you mean expansion tank? To relieve pressure?
Expansion tank goes on cold water line before cold enters tank.
See image:
http://waterheatertimer.org/images/water-heater-expansion-tank.jpg

Expansion tanks are installed when TP valve is constantly drizzling hot water due to very hot temperature settings. Or when water pressure is high, etc
It's also smart to install expansion tank so plumbing lasts longer, and older tank is less likely to rupture and cause damage. Inspect tank for leaks each 6 months.
Some regions require expansion tank by local code.
Water heater can be damaged when pressure exceeds 80psi. : Test pressure using pressure gauge at water heater drain valve.

Other resources:
http://waterheatertimer.org/9-ways-to-save-with-water-heater.html
http://waterheatertimer.org/How-to-adjust-temperature-on-water-heater.html
http://waterheatertimer.org/images/BTG-100-Pressure-gauge.jpg

Question is unclear.
What is a vacuum valve?
You posted under electric water heater.
Water heaters are associated with TP or 'safety overflow' valve, water shut-off valve, tank drain valve, pressure-regulating valve, mixing valve and possibly a few other plumbing valves.
Vacuum valve implies that a compressor is creating pressure on one line while creating vacuum on another line. I don't know of a water heater that uses air compressor.
Open following link to help troubleshoot leaking water heater:
http://waterheatertimer.org/Leaking-water-heater.html

Very simple - call a Plumber. He will fix your plumbing problem, and any foreseen potential problems related to your water leak. He may also suggest replacing your 6 year old water heater. Of course ..... for a professional cost.
However, depending on your time, skills, proper tools, patience and luck, it's easy enough to perform yourself:
1. Switch off the water heater Breakers (220v) in your service panel.
2. Turn off the water supply to the water heater.
3. Attach a water hose to the drain faucet. ( a good time to flush sediment -calcium deposits)
4. Open a hot water faucet to vent (above the level of the water heater drain - kitchen sink)
5. Remove the suspected leak components. (a union connection is a good place to start)
6. Install New pipes -3 wrap all male threaded ends with teflon Tap. Tighten securely - using 2 wrenches.
NOTE: Before refilling - you can turn on the water heater supply valve for a few minutes, to flush out any lingering calcium debris.
7. Shut off drain valve - remove hose. Shut off venting hot water faucet.
8. Turn on water supply valve to fill water heater.
9. Open a hot water faucet to relieve air pressure while filling - preferably the bath tub faucet, it doesn't have a screen that might clog.
10. When water flows without air - shut off tub faucet.
11. Look for leaks.
12. Switch the water heater Breakers on.
Note: 50 gallons should take an hour or more to heat up. Turn on a hot water valve momentarily within this heat up time to relieve thermal expansion. ( the PT valve on your water heater will perform this task if pressures/temperature exceed new water heater tolerances - it's just a precaution to introduce these fluctuating pressures on your fragile 6 year old unit)

These are just basic instructions - assuming you fill in the common sense and work safe steps.
Good Luck - The Skill will only come after trying.

Water heater should be set in a drip pan.
Otherwise, put 1 gallon bucket under TP release pipe to catch water.
http://waterheatertimer.org/Water-at-bottom-of-water-heater.html#drip-pan

When you say relief valve, I assum you are talking about brass valve located on top of tank or near top of tank. This is TP valve and a pipe connects to this valve and runs down to within 6" of bottom of tank. TP valve has a small lever on it.
If you are talking about tank drain valve located at bottom of tank, then add a comment.
Following information is for TP valve.

Dripping TP valve can be caused by 3 different thing:

1) Obstruction inside TP valve. Open TP valve for a moment to flush away particles and slowly lower lever back down. See if water stops dripping, if not, then replace TP valve

2) Defective TP valve: replace TP valve. Turn off electricity to water heater. Let heater cool for several hours so you do not get scalded. Close shut off valve located above tank on cold side. Open tub spout to relieve pressure, open tank drain or open TP valve and empty water below TP valve level. If TP is located on top of tank, not much water has to be drained. Tank is under press when hot, so be careful. Put teflon tape on new valve and install with big pipe wrench by turning clockwise until TP valve is very tight.

3) Overheating tank. TP valve is temperature-pressure valve. If tank water is over 150 degrees, the TP valve starts drizzling hot water to relieve tank pressure.
Lower tank pressure two different ways:
a) turn down thermostat
http://waterheatertimer.org/How-to-adjust-water-heater-temperature.html
b) install expansion tank on incoming cold water line
http://waterheatertimer.org/Water-at-bottom-of-water-heater.html#expansion-tank

It is possile that it had apressure fluction for various reasons. It's also possible that valve spring is just getting weak. What i would do first is replace T&P valve and see what happens.These are common 3/4 inch male iron pipe t&p valve available at most hardware stores. First shut off water supply to water heater, then open valve on hot water side of nearest faucet to relieve pressure. Remove old valve by turning counterclockwise. You may need a pipe wrench as they can be pretty tight. Remove old t&p valve and use teflon pipe tape or joint compound on new valve threads. Install new valve and tighten securely. Close faucet previously opened to relieve pressure,then turn water supply valve to water heater. Check for leaks and tighten further if needed. Check valve occasionally for next couple days. If no further leaks from valve then your're in business. Hope this helps. Thanks

This is not an "overflow". It is a Temperature/pressure relief valve. Sometimes they leak with age or build up solids in valve preventing them from closing completely. This is the single most important safety device on your water heater. If one of the thermostats is malfunctioning, the TPR valve will relieve. If the pressure rises too high, it will also relieve. You can replace the valve, but be sure it is the same temperature and pressure rating. You will need to turn the water off and drain some of the water prior to removing the valve. Don't forget to reinstall the down pipe that directs floor to floor or drain. If a new one leaks also, suspect your temperature control. Water heaters can be bombs if the safeties are not working properly

What is the leaking pvc pipe connected to? The T & P valve on the side (or top) of the tank? Or is it an incoming cold or outgoing hot water line? If you have just lit the water heater, the T & P valve may drip, but only for a little while and it will stop. The T & P (temperature and pressure relief) valve may be bad and need replaced. Turn off the incoming cold water valve and turn the gas valve to off position. Then drain the water heater using the T & P valve, it will stop draining when the water level is just below it. Remove the T&P valve and replace with new one. Use thread sealing tape on threads. Or call a plumber if not sure.