If you can't reach your doctor, your pharmasist will give you the best advice. Having worked in a pharmacy, but not a pharmacist myself, I would say it's okay to start the Vicodin now after 6 hours. One important question: How many hours were you supposed to wait between doses of Percocet? If the time has elapsed, and you'd now be ready for another Percocet, I'm certain it would be safe to start the Vicodin now.