Even after you've had a vasectomy your body still produces sperm. All that has happened is the delivery line has been broken. I had a vasectomy and 20 years, or so, later I had sperm taken direct from my testicle. This was done at the IVF Nawabutr Clinic in Bangkok. After 20 ish years my/your sperm is still there but the tail becomes deformed due to your body attacking it as an invader. The tail is removed and the head of the sperm is injected directly into the egg. How doctors explained it to us.
Cutting a long story short my wife produced 15 eggs of which 11 were fertilized. Those fertilized eggs are graded and used accordingly. The outcome for us was a daughter, on first attempt, followed 18 months later by a twin boy/girl, 3 in total children.
A few posters on here talking rubbish.