Is there a way to share the user database with another vanilla based forum (both with the same version)?
there isn't a separate database for 'users' it's a collection of tables within the database for the forum..
however, with some creativity and panache, i bet you could figure out a replication scheme that allowed you to keep the two databases in synch... but it'll be tricky and require a fair amount of coding to do.
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I will give it a try