Maybe she's feeling pain and it's making her uncomfortable during intercourse. Her using a vibrator could just been a matter of convenience because a female does not need to insert the vibrator to climax. U may want to have a dialogue with her. U can also increase the amount of foreplay. More foreplay means she'll create more moisture. Which will help if she feels pain during intercourse. U can also try lubricants like KY. Good luck!
Maybe sexual insertion is painful for her. A vibrator can be used from the outside only, rather than penetrating, and it gives a fast orgasm.
Why dont you ask her why she masturbates with a vibrator every night?
Nobody on here is going to know.
BTW, what is ISD?