SST_SWAP_EN_WAIT_ON_VSYNC is just setting vsync on/off.

FX_GLIDE_SWAPINTERVAL is about vsync swap interval and unlike the former, it only affects Glide applications. Setting it to 0 disables vsync.

with the monster 3d and diamond 1.10 drivers, SST_SWAP_EN_WAIT_ON_VSYNC=0 has no effect, at least for glide games. but one useful thing is to pass these variables via batch file, allowing to change settings without having to reboot, pandemonium demo comes with one for setting gamma to 1.0 (and gex2 recommends the same thing). it's odd that 1.0 seems to match voodoo3, which IIRC is running 1.3 for games by default.

with FX_GLIDE_SWAPINTERVAL=2 for a 30 FPS limit and pandemonium, i've noticed performance issues - just standing in place when the demo level starts, the MIDI will slow down a bit. input lag is pretty bad in this game with vsync in general, need to see sometime later if this can be improved with a V2 and triple buffering, because i don't think that was a thing on V1 yet. to my understanding the old method of triple buffering cuts down on input lag whenever FPS is lower than refresh rate, but when it matches the refresh rate it actually adds input lag compared to regular double buffering.

the 2.16 reference driver's control tab comes with toggles for vsync in D3D/glide. the way it works in that driver at least, any gamma/vsync settings for D3D are purely done in that tab; it will then ask to write the respective environment variables for glide into autoexec.bat, and when selecting no will write them into a seperate .bat file. so i'm still not entirely sure about SST_SWAP_EN_WAIT_ON_VSYNC, at least for D3D it has no function in this driver either.

edit: there is even a 3rd variable, SST_VSYNC - did not test yet if it does anything.