The vagina has a natural tendency to push out the dilators, especially longer dilators with a heavier base attached to them. The glass dilators you can obtain from Crystal Delights, https://crystaldelights.com/shop/dilators/, are much shorter, and for some women, stay in easier. You could try using a pair of “spanx” type undergarment to hold the dilator in place, but I’m not sure how comfortable that would be to sleep in.
For some women, sleeping with dilators is not essential to full resolution of their vaginismus. So if you find sleeping with the dilators too difficult, I recommend dilating at night and in the morning for a good 30 minutes each session.