You should contact the court house where the judgement was filed. They can help you with the steps to have it vacated. I'm not really familiar with the process and I know it can vary from court to court.
Any ideas on where I would start?
Multiple ways to go
MY state, will vacate upon payment if you file a motion. I live in Illinois. But most states don't do this
One approach is to contact the plaintifs attorney and ask them to file the motion to vacate. You will likely have to pay their court costs, but they often will agree. You have already paid it so they have no incentive not to work with you unless they are being difficult. They might bill you for their time too, just be upfront as ask them.