To use the Viper's extended modes in DOS, you either need special drivers for particular software, or a VESA driver. The VESA driver provides a standard interface to read and write to the video card in extended SVGA modes, and is the only real way to take full advantage of the Viper under DOS. The VESA standard has gone through various versions - 1.0, 1.1, 1.2, and the recent 2.0 extensions.
The driver diskette that came with the card has the vprmode utility which can install a VESA driver with the command vprmode vesa or vprmode svesa on newer versions. If you have lost your driver disk, you can get the latest one from the Drivers section at the Diamond Multimedia Home Page or from any of the mirror sites on the previous page.
Frido Garritson has written a replacement for the Diamond Viper VESA driver. It requires less memory, allows you do create your own modes, easier configuration of VIPER.INI, and allows double buffering in Flight Simulator on 2MB cards. He is only available via Compuserve, but you can download the latest shareware version of VPRVSH.ZIP from my account. This version supersedes older versions which can be found on some FTP sites.
SciTech Software have a Universal VESA 2.0 driver which supports nearly 200 cards including:
The has now been expanded to a larger package called Scitech Display Doctor 5.3a. The 5.3 version crashed my Win95 sessions upon installation, even if I gave it ALL the card details and bypassed the autodetect. (I have not tried the 5.3a version with or without the latest Diamond display driver). I believe that this 2.0 driver still requires a 1.x VESA driver to be loaded first.