What happens when the judge grants me a default judgement for divorce ,and then ex files for a motion to vacate judgement?

Then you respond to her motion in court, you show how she was properly served and did not respond or show up.

And then it will be up to the judge to decide on the issue

These are typically granted unless too much time has passed (>6 mo. For example). But there is usually not much to be gained in a default as it is. You don't get all the property awarded to you just because it's a default. The only benefit is there is no opposition to anything you allege and request.