I actually have played with a few old VFDs. I didn't make any real
measurements,

http://www.mif.pg.gda.pl/homepages/f...27/1/12DL8.pdf

The first graph on the 12DL8 pdf will give you a good idea what it's
operating characteristics is like. The current scale, and voltages are off,
but it will give you the general idea.

but I pushed the plate to the full 400v of my power
supply with no ill effects. The only limiting factor was power
dissipation. When the tube got hot enough, the glass cracked.

That would be easy to solve. Cover the anode side with thermal grease, and
use a clip to mount it to a big aluminum heat sink. You would have something
that would look almost like a solid state monolithic amp. :-0