I'm no teacher but I think a vicious circle is a situation in which your actions to solve a problem leads to another problem, and when you try to solve that second problem it worsens the first problem.
E.g.: you lose the rent money at the blackjack table. You ask a friend to lend you some, and you play again some blackjack to get the money back. Unfortunately (so to speak), you lose it again, and now you ask another friend for even more money to win back all the dough you lost. Etc.
A virtuous circle is the opposite. E.g. the more you train, the better you do, the more you're motivated and the more you train etc.