I have a really expensive DVM with a VFD display that is getting very
very very dim.

A new one from Noritake is $800, so that's a no-starter.

Any ideas how to pep up a fading VFD display?

it's the 19-segment alphanumeric type, one line, 20 characters.

( It looks identical to the ones on Gilbarco gas pumps. )