I agree with Jaz, it's verrry unlikely the vasectomy is off.. honestly, they're very effective and fail once in a great while, and that short period was most likely implantation bleeding!
the 1st guy. Becuz sperm can reach and egg in 30 mins, but penetration depth and where egg is in ovaulation cyle are factors to be consider. so around 14 days after possible conception which i give to the 1st guy becuz though its possible to still father a child with a vasectomy its more likely the one time on the 9th
DNA test. Not to sound rude but no one can tell you, even doctors, who the father is.
You could get his sperm checked to make sure because sometimes those dont work.... but other than that you dont really have a choice of knowing until after the baby is born and doing a DNA test.