You've got some really practical advice so far, that ITA with.
Also, consider what you're life will be like if you don't resolve your trust issues and the child IS yours. And, what will happen to the relationship between you and the 6 year old whom you've "raised from birth."
I became pg 6 yrs ago after my husband's vasectomy, but he had never gotten retested and needed a second vasectomy before his test was "clean".
If you have had unprotected sex, then you need to be tested for the HIV virus, and also for other STD medical conditions.
get a sperm count, and if you have any, try a little trust
good luck
Go to the doctor and get a sperm count. That could clarify things for you quickly. If you are deemed entirely infertile, well, you know what's going on. But, just because you didn't get anyone pregnant for 17 years, does not necessarily mean the vasectomy worked. That's why it is important to get get your two sperm counts after a vasectomy! Also, vas deferens have been known to "heal" themselves and grow back together - very rare, but it has happened. So, you may have some swimmers in there, even though it seems impossible. Do the sperm count before you make accusations or judgments.