Again, It is NOT true that you can't file a motion to vacate. The party who lost is the one who would be filing such a motion. So either you misunderstood something or they did.
You have to know who is suing you. The plaintiff is listed on the summons.
What court issued the summons (civil, small claims)?
If you have a copy of the letter, you MAY be able to get the judgment vacated. You can show the letter and explain in your motion that you were under the impression that the notice of a hearing meant you did not need to do anything more until the hearing.
It may be allowed, it may not.
If the court clerk does not allow you to file such a motion because you aren't the plaintiff then ask him to cite what statute or rule states that. Tell them, your research shows that the party who the judgment was applied against has the right to file such a motion.