Well I see this can be done in 2 ways:
1) grab shitloads of proxies and do it manually with firefox and foxyproxy. Maybe you can find a plugin that would rotate the proxies when you press a button or something...
2) This an easier way if you can code - figure out how the button works. I assume it's with a POST request. Write a script (yeah Python would be sufficient for this) that sends the exact request from one of the shitload of proxies that you should have and bam! instavote right there. When the request is sent, make it pick another proxy from the list, wait a few minutes and send the same request again...