Where to include a new variable?

[Opserty]

Have you tried $post['postuserid'] instead of just $post['userid'] maybe that will correct it?

Also you can just say this for your query:
[sql]
SELECT COUNT(threadid) AS num FROM vbthread WHERE forumid IN (57, 58, 59..., 63) AND postuserid = $post['postuserid'][/sql]

Finally remember you are running a query on every postbit on every showthread so you may want to rethink what you are doing. You may be able to modify the showthread query using the showthread_query hook and add a join/count to the query.