Vandals have what is known as sock-puppets, multiple accounts in order to vote several times. Obvious ones (such as those who voted in their first few edits) are caught, but there are much more sneaky ones. There was one such user who made over 12,000 edits before being banned as a sock puppet of another banned user, but most of the time, editing for a few weeks with another account will make a convincing sock puppet.