[Marco van Herwaarden]

WHERE vb3_pollvote.userid IS NULL
AND vb3_pollvote.pollid = 240

That will never be true. Either the userid is null (ie. no match) or the pollid has a value, never both at the same time.

Try:
[sql]SELECT vb3_user.*
FROM vb3_user LEFT JOIN vb3_pollvote ON (vb3_user.userid=vb3_pollvote.userid AND vb3_pollvote.pollid = 24)
WHERE vb3_pollvote.userid IS NULL
AND vb3_user.usergroupid =26[/sql]