If I have a vasectomy will fluid build up in my scrotum every time I have an orgasm?

No way. Only 10% of the ejaculate (fluid that comes out at orgasm) comes from the testis. Most of it comes from the prostate and seminal vesicle. After the vas is sealed, the production of testicular fluid is decreased. You will always be making sperm but they have nowhere to go and they get reprocessed. Nature is very efficient.
Vasectomy. When a vasectomy is performed, the vas deferens is cut, and then tied, clipped, folded and/or cauterized. The semen does not collect in your scrotum with each orgasm. Occasionally, a sperm granuloma can form, but the body usually reabsorbs this buildup of semen over time.