We had pretty good luck keeping the old and the new separate. Once we migrated all the files into the vault, we make the network locations read-only for 1 year. Then we removed access to the network locations completely. We still have all the data, but not accessible to anyone but IT.
In my opinion it should be all or nothing. If you don't force everyone to use the WPDM you will run into file management issues as well as process issues.
We knew our data was a complete mess, so we just copied everything into our vault as is. We have a "Legacy" folder, "Projects" and a "Production" folder. All Pre-go-live files were copied into the "legacy" folder. All new designs would go to the "production" folder or the "Projects" folder. We did that for a number of reasons, the biggest reason was to denote any file in the "Legacy" folder had to get updated to before it could move over to the production folder. That actually worked out pretty good because the files only got updated if a revision was made to them.
I do believe our key to success was a full switch over and to force the new process.
Thanks very much for the reply;