It’s not likely and very very rare after so many years for it to fail . Are you worried a vasectomy may have failed ?
There is such a possibility, *if* his sperm count is not zero. That could happen if the original vasectomy didn't work, (or I guess if he was fibbing about having had a vasectomy at all), or if the vasectomy failed years later.
After a vasectomy, guys usually get their sperm count retested at the 3- or 6-month point so they can be sure the vasectomy worked. Did the guy do this? If the guy never was tested after the vasectomy, it's possible that he doesn't know if his vasectomy was effective or not.
Another possibility would be if someone had a vasectomy and then later his tubes (or one tube) managed to reconnect. It seems like it would be rare to have happen, but the medical literature says it can.
Simplest thing to do is to ask the guy to go to his doc or urologist and get tested to see what his sperm count is at this time. If it is zero, then you have your answer.
The ultrasound suggests your ovulation was close to May 11th. Sperm from either encounter might still have been viable, May 7th more likely. You have eliminated May 5th. It’s time to ask May 7th if he will consent to testing. Hope I have the details right. If he won’t consent, you could get an Ancestry test done to see if you can tie in to a relative of his.