What voltage is the dc adapter?

You will need at least a 4 amp rated regulated 12 Volt power supply.
These are commercialy available from most electronics parts suppliers.
They come on a circuit board. You can mount it in your own case and
use the original Viewsonic wire end. If you do a google search for 12
volt reglated power supplies you should find many of these.

However, with a second-hand Sprint AC adapter with 3A,
12v adapter, the screen blinks on and off
intermittently. Obviously there is some difference here between a class
2 adapter and a power supply box.

My viewsonic va520 had the problem of going of after about 20min,then if unplug the power supply,plug back in,it would cut out every 5 min. I have an adapter which is not for the va520,It`s a liteon model pb-1240-01a-rohs,12volt at 2amp also tryed 12v at 5amp; by dynex model bt60p1205, and they both work it. found my problem to be a couple of capacitors on the main board.,(c1 and c9.,replaced those now all ok again. hope this helps someone.

I need a power cord for va520

The dc adapter should be rated 9.3 amps at 12 volts DC, remember that under usage in an AC system the amps are 17-21 amps and the voltage is 110-120

Look carefully at the jack for the power input; the required voltage and cuirrent is normally shown either molded in the plastic or printed faintly near the jack.
If it isn't present, do a search for reviews; many of them will include the either the input specs of the monitor or the output specs of the provided adapter.