I always heard it was recommended to set it to only the half of your Vram but this tweak exist since ages, probably they never had in idea that some GPU's will have up to 6 gigs of Vram like nowadays when those tweaks were published, maybe you can push it a bit.
RO2 is not like Arma 3 who is able to eat 2.4Gigs or Vram after just 20 minutes of gameplay i got a 3G Vram GPU, got mine set to 1536 in the ini and if i recall correctly RO2 never eat up so much, pretty sure i was far from this 1.5g limit I set, I'll have to monitor this again with MSI-AB to get accurate data as i don't recall exactly the values.
I wanted to get rid of the streaming loading textures and have the vram loaded at max to avoid the poping up effects but i never could get it done without some gfx corruptions in game so i gave up trying.
