Two things here.
-If the vote is 8-7, then switching one vote over will lead to a 7-8 vote. I count 16 clan heads, with 7 casting 'yes' vote; I assume two others voted and just weren't shown in the comic. If it's 9-7, then it becomes a tie, and I assume in the case of a tie Dvalin gets to cast his vote how he pleases.
-You're assuming that the clans cannot change their vote once cast. And even if they couldn't normally, I imagine they can when it's clear there was vampiric influence on the council's vote. And it's unlikely that the vampire will be able to act with impunity on this second vote. All they need to do to get rid of the domination is walk out through the orange barrier and then back in.