Hi , you can divide your obj model into sections by using g parameter:
g -_0
v ...
v ...
v ...
f ...
vt ...
vt ...
vt ...
g -_1
v ...
v ...
v ...
f ...
vt ...
vt ...
vt ...
I think your vr painting app must provide vertex paint feature.
Obj file format officially does’t support vertex colors, but many applications are using it by adding colors as R G B A after vertex coordinates. But Epic Games didn’t implement this feature into engine, which means you should use another file format like FBX if you want to import exported files with Unreal Editor’s importer, or another app that doesn’t support vertex colors from OBJ. FBX is harder to implement but it is better than OBJ.
You can represent your interest into this issue if you want. I created a request topic for this:
https://answers.unrealengine.com/questions/491053/request-import-obj-files-with-vertex-colors.html