Here is where I drew the line in my reply: a VTT is any program in which the players control their characters and one player / the DM controls all other game relevant creatures (e.g. NPCs and enemies), while the computer itself controls none of them.
I know this overlaps with some game territory, but not with much, and that is the price you have to pay in order to take games away at all.
Alternatively, express in the OGL that you limit your VTT to the limitation of the VTT policy, and if your VTT offers any feature not in the policy, this implicitly makes that feature available to all other VTTs using the VTT policy.