1990 Kawasaki VN 1500 jumps in First gear

clutch plates need replacing , I would replace the steels as well and also change the oil and filter as it will be polluted with clutch dust.
Would also help to flush clutch fluid as well.

If it doesnt jump out of any other gears. It may even grab 2nd on decel. sounds like a bent shift fork.

your clutch is in need of ajustment or need clutch work on.

It sounds like a bad shift fork, shift drum or worn dogs on the gear itself. Very labor intensive. have remove engine from the frame and split the cases.

Check your motor and transmission mounts. Not much into the slave cylinder, but on hydraulic cylinders there is sometimes a run-out where the hydraulic pressure is too weak to do the job. This sometimes happens with the shaft leaking internally and allowing pressure to bypass the valving. But you have a different one, is it good?

Consider a shock absorber. The first push it can make, then it does not refill the void fast enough to make the second push. Because of engine reving, you jump from 1 to 2 quicker than 2 to 3 gear. The slave cylinder does not have enough time to refresh. Maybe look at the weight of the oil in the slave cylinder.

It is under the left engin cover. You have to remove the shifter and the footpeg or floorboard too. Then remove the 4 or 5 allen bolts and the cover will come right off. No oil or anything will leak when this cover is removed. You will see the clutch slave cylinder behind this cover. I found the best way to bleed the clutch is to fill the master and open the slave and let it drip for a while (gravity bleed) Do not let the cylinder at top run low while you are doing this. After a 10 minutes or so, tighten slave then refill cylinder and do the old pump and drain bleed a few times. I found that it works better with the cover installed on the cylinder when pumping to bleed clutch.

if your oil is too thick , your clutch will sleep and go away .
if the clutch is old , you obviously will have the same problem .
please explain your problem when you ride

sounds like the gear selector plate plunger is worn or needs a new spring. on most engines a hollow screw @ 3/4" is accessible from under gear box, take it out and stretch the spring and retry to eliminate possibility.

http://www.bikebandit.com/houseofmotorcycles/kawasaki-motorcycle-vulcan-88-vn1500a6-us-1992-clutch/o/m4620sch62090

this schematic list the brake breather. I hope this helps.

try motul 5100 engine oil i have found sum mineral oil can make a cluch act this way