I'm on 30 mg vyvanse for a month now. Having trouble sleeping at night (not too terrible but enough). I want to take Benadryl (diphenhydramine) to help at night. Isok?

Yes. No problem with taking Benadryl (diphenhydramine). HOwever if your problem with sleeping is from nasal obstruction, then nothing would work that well. Consult an allergist if this remains troublesome.
Yes, it's ok. While Benadryl (diphenhydramine) does not really reverse the stimulant effect of the Vyvanse, it's main goal of sedation may help you sleep. The two medications work by different mechanisms and are safe together when used the way you are describing. Many people feel a little "hung over" from Benadryl (diphenhydramine) in the morning. It may be worth considering prescription sleep aids such a trazodone to help you sleep.