From what I understand the entire company works on games now. Just one office spearheads the project.
It all depends on if the game is behind schedule...
All the studios fall under Rockstar Games and thus do cross over to assist on portions of a game but they don't all work on one game shelving all other projects...A portion of Rockstar North did assist with RDR because the Housers were having trouble finishing the game...They asked Benzies (former GTA lead developer and designer) to assist with finishing the game...
I wouldn't worry about VI being delayed because of RDR2...
I would be more worried about this being the first 3D GTA game without Leslie Benzies involved...