below is the response I got from the dentist when I emailed them. Im not so sure about the vacuum bit, I just wonder if they are out to make a new splint ( expensive ) another dentist I went too just said that they would alter the splint to fit.

"with any fillings we cant give a guarantee that existing splint will fit as they are vacuum formed on a replica of your teeth so if the shape of your tooth changes there is no exact way to make it identical. It would depend on the size of the filling needed and the position of it . Please bring your splint with you to your appointment and we will do our best to try and make it fit, but if the filling is considerably large you may have to consider a new splint but we wont know this until we see you and make the assessment."