My wife thinks shes pregnant even tho i had vasectomy in march and 0-sperm semen analysis in july. should we have waited longer to have unprotectd sex?

Not really: It is possible that even with no sperm about 4 months later, that you could have developed a failure. Unlikely though. If there is sperm now, then i guess that the previous sample was misleading. I suggest that a semen analysis check and a blood test for pregnancy are in order. Time will solve the question of pregnancy. Get tested when you are worried enough to invest in the test. Best wishes.

Possibly: It depends when you had unprotected sex. After traditional vasectomy, viable sperm may persist in the ejaculate for several weeks. In a 2003 study performed in mexico, 17% of men had viable sperm present six months after vasectomy. The safest thing is to use protection until you have confirmed azospermia on an official semen analysis.