Apologies if the bump is no longer relevant, but I have myself just found out Legacy runs Heretic very smoothly in VESA resolutions.
I was using 1.4.0 DOS build but in 1024x768 and it worked superb, I only noticed the lighting was somewhat brighter than in the original, especially noticeable at higher resolutions, even with brightness at minimum, but overall Legacy turned out to be such a cool port, playing Heretic with it is a pleasure (as is Doom by the way). It does not seem to support Heretic demos, though. And it seems that by the time Hexen support was complete DOS version was no longer maintained.
Alternatively I tried HeXetic build available on the Internet Archive and it was able to run Heretic in higher resolution. According to the archived home page it should also run Hexen, but I did not figure out how to do that. Not sure if it runs demos either, and you are locked into full screen view with its custom alternative HUD only.
If you want to run Hexen in DOS in 640x480 resolution I suggest trying hack-of-a-port of my own. It supports 640x400 and 640x480 resolutions using the method @gerwin has used for MBF 2.4 (i.e. with horizontal "pillars" in 640x480 - and it actually uses code that @gerwin authored for that), supports demos, should be save game compatible and includes quality of life improvements, like free mlook with wider vertical range, going back to players's save game when dying as opposed to game's own autosave, more sound channels, additional savegame slots, simpler mouse interface for flying and such. You can get it here and source is available on moddb.
GA-G41M-Combo G41/ICH7 - Core 2 Quad Q9550 - DDR3 1033 - Radeon RX570 - YMF744 (Cobra) - X3MB (Buran)
Beetle/M/i815+ICH2 - Celeron 566Mhz - Opti 924 (Typhoon Media)