MoJoe,
Retention has nothing to do with it. The only reason I can see you want to use multiple VSG is if you have a company with separate legal entities that use DA. You can then separate the DA nicely by using VSG. Other than that why would you want to separate different VS into separate VSG? It makes no sense..
I have only got 1 customer that uses more than one VSG, but that's because they a bank and wanted a single site but wanted to be able to split it in DA.
This is how it looks like:
VSG - Legal Entity 1
VS - Country 1
VS - Country 2
VS - Country 3
VS - Country 4
VS - Country 5
VSG - Legal Entity 2
VS - Country 1
VS - Country 2
VS - Country 3
VS - Country 4
VS - Country 5