How do I vacate a default judgement?

You'll need to file a motion to vacate the judgment, and you need to do that as quickly as possible.  Based on your question, it sounds like you've got a good argument for this, and that's relatively rare, but you need to be prepared to explain, in the motion, why it is that the case needs to be tried, what your defense will be.

The courts place a very high value on finality, on avoiding having cases go on endlessly.  So, whenever you find out about a court decision against you, anything you want to do about that has to be done quickly.  If you sleep on your rights, you will lose them.