about 5 weeks ago, my wife and I desire to have one more child. Supposedly, sperm can be present downstream and in the system for some time after the vasectomy
If there are still sperm present, which is doubtful, if there were anything wrong with them the egg probably would not accept them to conceive. If they were "bad" and you did concieve (conceive), it would likely not implant or make it very long. Nature has a way of taking care of imperfections.
Talk to your dr, I believe there is a way they can "extract" semen and do IUI. Your dr would know the best route.
The sperm wouldn't be chromosonally defective, although they may be too weak to fertilize and egg. It takes a few months, (typically) for your sperm count and motility to build back up again. Good luck ttc, I hope you have a pregnancy very soon.