Consider looking at the
"Ice Tube Clock" which uses an IV-18 and
MAX6921 serial HV driver, 20 output. It's similar to the HV5812, or Microchip
HV518. I think the
IV-21 VFD is the same except for 8 digits vs 9 with the IV-18.
The Adafruit kit version was not so great, there were issues and then it got abandoned.
The P-ch mosfet switching the tube's filament power ended up giving low voltage to the heaters which was a huge PITA.
It uses the MEGA328 MCU to generate the boost-converter 31kHz but it's open-loop, no regulation.
Also uses the MCU as the real-time clock, so it drifted a bit too much and ate batteries when on them.
There are a few different firmware loads out there. People upgraded to a decent RTC or GPS, and better dimmer etc.
Source code is open, maybe look at
https://github.com/johngarchie/xmas-icetube; mods for HV5812 would not be too difficult in display.c
The cheap plastic case was too light, with tension of 5V power cord it slid off the desk and smashed the tube into pieces. I have a replacement but the clock was just not worthy of more time.
If you're going to have fun doing this project, do it right with a decent RTC and enclosure.