She is not staying in the royal vault, after everyone leaves she and Phillip will be raised back up and taken to where her parents are interred, or so I have read
She will be buried in the King George VI Memorial Chapel vault. She had this addition built on in the 60s at her own expense. It has space for 3 monarchs and their spouses. George VI and his wife are already there. Margaret was cremated so that she could be fitted in with them. Prince Phillip was placed in the royal vault, but will be moved to be placed next to the late Queen. They’ll all be together.