If you're using the default D3DDrv that UT99 first came with there's two options "UsePageFlipping" and "UseTrippleBuffering", but I don't remember if those actually properly communicated to the GPU to do what is essentially "vsync" or not. Turning them off should dramatically speed up the rendering process though because it wouldn't have to wait for those operations all the time.
It really depends on the hardware you're trying to use, graphics cards in the 90's were pretty wild west at the time. D3DDrv is notorious for not properly functioning on some of those cards even.
If you're using Glide though, try seeing if "FastUglyRefresh" set to true does anything, but it'd probably be up to the Glide device drivers themselves in whether or not you want vsync enabled, and not actually UT99 itself.