I have mentioned this before and it has fallen on deaf ears. You really, really, really need to make an option on the server that prevents users from exclusively checking out mergeable files.
Out of the box, VS and Vault will enable exclusive checkouts even with the enhanced client. This is a serious pain. It is incredibly difficult to herd developers stuck in the old style of work into not checking out files exclusively. Undo check is a last ditch emergency solution. You should simply create an option that throws an error back to the client if they try to exclusively checkout a mergable file.
Prevent exclusive checkouts
Moderator: SourceGear