To be technically correct, it's called an "Ectopic" pregnancy, which occurs when a fertilized egg begins to grow in one of the fallopian tubes that normally transport an unfertilized egg from the ovary to the uterus rather than descending into the uterus and attaching to the Uterus wall/lining.
As for your question, a man who has had a vasectomy is by definition sterile and cannot impregnate a woman (assuming that the vasectomy was successful, of course).
Vasectomy involves cutting and cauterizing or clamping the tubes that transport sperm from the testicles, so after this procedure has been performed, no sperm will be present in the ejaculate that comes from the man during orgasm.
Normally, only a few small drops of the ejaculate come from the testicles anyway, the rest is from the prostate so there is no noticeable difference in ejaculate from vasectomized men.
For several weeks following a vasectomy, birth control measures should still be used to prevent and remaining sperm from potentially causing pregnancy, but that sperm wouldn't cause an ectopic pregnancy any more than other sperm.