I have a sneaky suspicion that I'm out of luck for VESA compatibility in DOS with a Voodoo5, but I'm just wondering if anyone knows for sure.

For example, TES Legend : Battlespire  uses high colour 640x480 VESA mode, and will not work on V5 but works just fine on an old ATI Rage (with Mach64 vga bios).  

Did 3dfx ever release any type of VESA patch or driver for its 2D/3D chips? If not , then I'll be forced to finish the cooler for my Obsidian2 X-24 (and yes, I do know what the Quantum cooler looked like and how it was installed) and pair it up with a true VESA vga.

Find Voodoo Heaven here ...
http://www.thedodgegarage.com/3dfx/index.htm

The Voodoo5 has full VESA 3.0 support in BIOS, so the Problem is likely a missing mode or the Game not being able to handle the VESA 3.0 modes.

Since it's fully supported in BIOS already since Voodoo3, there are no TSR Drivers.

Your best bet would be either to try other Video Modes or (if that doesn't work) actually use a non-3dfx Card.

The actual error that I get is "Cannot set 15 bit 640 x 480 mode"

Sounds like the 3.0 VBE bios in a voodoo 5 is not  fully compliant.  The game itself doesn't offer any other modes that I know of yet, but there might be a command line switch or something.

Otherwise, do you know of any other external VESA 3.0  emulators similar to UNIVBE that can function with newer cards?

Actually, I just read a document that states to be VESA compliant, the hardware only has to respond to a GetVESAInfo command, which means the game itself is not VESA compliant by not choosing a display mode that the hardware can support.

The Voodoo5's VESA definitely doesn't have support for the very old HiColor (32k) 15bit mode, so I'm afraid you're out of luck (unless you can choose 8 or 16bit Color.

Well, I had the same problem, non working vesa based games, and somebody reply me that the problem was the NTFS file system (I'm using win XP). Don't know if it's true, but some games that used to work on '98, don't do now.

Yep, that could actually be if the Game relies on old DOS INT# I/O requests.

Old DOS installers also often fail when they check the remaining Hard Disk space and see values >2GB free, often leads to internal overruns and weird errors, for example.

Ja! seems like the old '98 is still useful. I'll have to create a partition for it.