How reliable is a vasectomy? My husband had one 2yrs ago, followed all post op directions, but I still insist on addl contraception. Am I overreacting?

Yes. If his post-procedure semen analysis negative, then there is no need for additional contraception. While not 100% effective (the "act of god" pregnancy can still occur), if negative then there should be no worries.
Very reliable. If a vasectomy is performed correctly, there is no need for additional contraception because it is >99% effective. He can go to his regular doctor for a semen analysis. If no viable sperm are seen on a sample, then you should not require additional contraception.
Yes. Once the post vasectomy semen is checked and confirmed to be 0, you can feel safe.
See urologist . A vasectomy is very reliable, and the ejaculation is tested afterwards for semen. But for your peace of mind, your husband could visit his urologist for testing.