640x480x256 and higher modes would not work on my similar card, a Diamond Stealth 64 2MB VRAM PCI. It uses the S3 Vision964 chipset. Bios version is v1.05H.
S3VBE20 loaded but didn't do anything, still no 640x480 in Quake. I tested all the UniVBE versions from 5 onwards and found 6.7 the best (it also requires no registration).
Benchmarks
Without UniVBE (Card has a default VBE 1.2 bios)
Quake = No option for 640x480 (No game at 640x480x256 or > runs)
Vspeed = 19.7 MB/s (BANKED)
With UniVBE 6.7
Quake = 20FPS 640x480
Quake = 41FPS 640x480 + FASTVID
Vspeed = 46.82 MB (BANKED)
Vspeed = 76.68 MB (BANKED) + FASTVID
Vspeed did not give LFB results, don't know why. I confirmed LFB is working though as enabling it in Fastvid doubled the frame rate in Quake.
UniVBE 6.7 was realeased August 2002. I also have UniVBE 7.2 for DOS but strangely it has an earlier date of Feb 2002 . It gives the same results as 6.7.
The speed increase with UniVBE 6.7 alone is over 100%. With FastVId as well this card performs as fast as my Geforce 4200ti in Quake. Benchmarks performed in DOS 6.22 on a PIII@500 on intel 440BX board.