I do not really get the problem.
My page has 2000 users,... I can just delete all my virtuemart data (refresh install). I have then still the users in vm, because there DO NOT EXIST special vm users anylonger!
What you need to migrate are the j15 users to j25, that is the only thing. And vm does anyway handle both, so hmmm. What yah doing? The only thing we migrate are the addresses.
So you can just use your old database and dont think about it. OR you start a new database, THEN you must indeed export the users from the old database to the new one, right.
To underline again. People are still shoppers, even when the vmusers table is emptied. Just some vm specific properties are gone then.