What is motion to vacate dismissal of divorce?

It helps to break it down.

Divorce we can take as understood.

Dismissal of divorce is a decision in which divorce proceedings are "dismissed", that is, stopped.

A motion to vacate is a request to nullify (make void, set aside) a previous judgement.

So, all in all, it comes out to:

There were divorce proceedings. One of the parties filed to have them dismissed so there would be no divorce. The judge did so. Now someone (possibly the other party, possibly the first party changed their mind) is seeking to have that judgment vacated so the divorce proceedings can continue.