With a vasectomy, you still ejaculate. Most of the fluid in an ejaculation comes from the prostate, which isn't affected by severing the vas deferens

Hmm, I wonder if it's really unaffected. Have they found a mechanism for the small but statistically significant increase in prostate cancer rates related to vasectomy?

Vasectomies don't prevent ejaculation. The semen is just sperm-free (after a brief period of time).