Will taking Ambien (zolpidem) for sleep make the 8 mg of Valium I take a day for pelvic pain, itching, and spasms less effective.

Don't combine both. Don't take both Ambien (zolpidem) and valium. Have your doctor optimize a single agent as they function similarly.
No. The Ambien (zolpidem) won't decrease the efficacy of the valium, but it is a lot of sedative medications to take, both of which are habit-forming, and i don't think it's wise to take them together, due to fear of exponentiating the effects.