In the posting_preview.html template file, you can find this, in the poll_option loop :

<!-- IF S_IS_MULTI_CHOICE --><input type="checkbox" name="vote_id[]" id="vote_{poll_option.POLL_OPTION_ID}" value="{poll_option.POLL_OPTION_ID}"<!-- IF poll_option.POLL_OPTION_VOTED --> checked="checked"<!-- ENDIF --> /><!-- ELSE --><input type="radio" name="vote_id[]" id="vote_{poll_option.POLL_OPTION_ID}" value="{poll_option.POLL_OPTION_ID}"<!-- IF poll_option.POLL_OPTION_VOTED --> checked="checked"<!-- ENDIF --> /><!-- ENDIF -->

The part name="vote_id[]" id="vote_{poll_option.POLL_OPTION_ID}" value="{poll_option.POLL_OPTION_ID}" (two times, for checkbox and for radio) is completely useless in the posting (preview) context, unlike in viewtopic.

The message author cannot vote while preview his or her text, before posting a new subject with poll, or while editing a subject to add or modify a poll.

If the input needs a name and a value attributes for somewhat reason, an expression like name="vote_id" value="" should be enough.

An alternative would be adding the possibility to vote inside the posting editor : I create a poll, I immediately vote and then submit the poll for other users.