I’m not sure how Render Region/Border works with Eevee regarding vRAM. I am also not sure how the composition would result due to the following limitation:
The screen space effects disappear when reaching the screen border. This can be partially fixed by using the overscan feature.
https://docs.blender.org/manual/en/latest/render/eevee/limitations.html#screen-space-effects
You try Pixel-precise render border addon:
https://blenderartists.org/t/list-of-addons-that-work-with-2-8/1132912/505
And maybe you should simulate that Overscan (Render tab > Film > Overscan) by choosing a render region larger than 1/4 of the frame (I’m really not sure what I am saying, I have not tested it)
Edit:
I have done a quick test and small render regions do not seem to affect vRAM usage or render time in Eevee.