The VRAM card doesn't affect Logic Board video at all.
The X100 NuBus PowerMacs all shipped with two monitor outputs.
One was the logic board video, which uses main system RAM for it's VRAM, is very limited in resolutions, is slow, and slows down the whole system because of RAM access conflicts.
The other output was either the A/V card or the HPV card (what I called VRAM card earlier).
The A/V card provided some video in/out capability and was kind of sucky performance-wise. It shipped in the NuBus PowerMacs with the "AV" suffix.
The HPV card, which shipped in the non-AV PowerMacs was basically just a fast frame-buffer, but it sits on in the PDS slot of the Mac, and it's really, really fast.
The HPV card that shipped with the 8100, the Radius NuBus machines, and the Power Computing Power 80/100/120 machines, has 2 MB of VRAM soldered down and has sockets for another 2 MB of VRAM. 4 MB of VRAM will give you a pretty nice range of resolutions and color depth.
The HPV card in the 7100 and 6100 only had 1 MB of VRAM soldered down and was expandable to 2 MB. It's much less nice.
Of course, by this time, when you find a 6100/7100/8100 or clone, there's no telling which HPV card will be installed. The cards have had loads of time in which to move around.
Oh, one interesting bit about the Power Computing version of the HPV card. It was just like Apple's 2/4 MB version, expect it had connectors for both Macintosh and VGA style monitors. There was a little switch on the card for choosing the active connector. So, if you find an HPV card with two monitor connectors, it is one of the Power Computing cards. And no, you can't use both connectors at once.