[Dismounted]

As long the MySQL user for vBulletin can access your other database (ie. is on the same server and has SELECT/INSERT/etc. permissions), you can use the "database.table.field" syntax.

PHP Code:

$vbulletin->db->query_read("
    SELECT *
    FROM database2.table1.fieldA AS fieldA
    WHERE fieldA = $someid
    LIMIT 1
");