In manufacturer if you try to insert just the required field and put a dup value you are able to insert the second record without matters. Nothing is perfect, but since the name is the only required field, in my logic no dup should be allowed in any case.
I think also vendors suffer the same problem. Not yet tested. I'm manually moving the records from old db structure. About this case, nice tool the import/export tool you did, but where is the import/export for vendors and manufacturers?
Since they are often required do you really want everybody reassign thousand of products manually?

Well in my case I've experience on db relationship and I'm fitting the need in old db with parent constraint, but a product like this that is developed as buy and run, should really offer that.

Bye
Andrea