Vote Parties give rewards when the server has reached a certain amount of votes! Vote party is held when the server gets 256 votes. That is when people vote on multiple sites. Vote parties can happen multiple times a day if everyone votes. To get rewarded you have to meet these requirements:
You must be online when the vote party is held.
You must have voted atleast 4 times on the day when the vote party was held.
You must have free inventory space to get the vote party rewards.