Actually they can. I've just tested this.
When you post the new thread you can define if you want to create a poll. Then when you submit your new thread you're taken to the poll options.
^ actually if you started the thread you should be able to do this by clicking "Thread tools" --> add a poll.
@ Sean. Vbulletin doesn't allow that unfortunately. We'll see. Maybe when your mod powers will be back you can do that. It's impossible for ordinary users.