My grandmother had hives and she told us it was caused by the heat (we lived in south Texas--a veritable furnace). So, in the 1950s, she and my grandfather got a ranch in Colorado to spend the summers there.
I can't imagine how a vasectomy could cause hives or any allergy unless your husbans contracted a virus of some type. After reading up on hives, it did say that hives could be a symptom of some other disease. It also mentioned that hives could be caused by some disruption of the auto-immune system.
If nothing is helping with the allergy, maybe you do need to seek out a specialist with skin allergies. And yogahoneybunny could be right about a change in diet. BTW, did the vasectomy work out okay and was there no infection that showed up? I was thinking of the change in sperm count and also being tested for white blood cell count.
Hope everything works out. Good luck. Bye