Depending on the state, there can a myriad of reasons to get a judgment vacated. Improper service is just one. A proper court procedure may not have been followed, and that may be enough. Exceeds usury interest limits, CA never responded to DV request or provided insufficient DV, CA never adhered to FDCPA §809(a), "I was unable to respond to this because ..." etc.
If the OP wants to simply pay it, well then OK. That's up to the OP to decide. But just as those who advocate just paying a CA will make them go away and besides you know you owe it so you might as well pay it is nothing short of a B.S. response, telling someone they just have to pay a judgment is also a B.S. response.