I don't think you understood the idea behind votecoins.

You think someone will buy all the votecoins and then vote to win all bets?

First: once someone does the chain is expected to die, so all votecoins become instantly worthless.

Second, in the original Truthcoin paper there is a safeguard mechanism in which all betters are expected to vote with their bitcoins. That will certainly be activated in case of a massive attack, so the attacker doesn't even get the bitcoins and loses all his votecoins.

If they try to make small attacks by voting just some small markets wrong this will still cause votecoins to lose most of their value.

So this is not expected to happen at all, it goes against all incentives.