What male reproductive structure releases sperm into the vas deferens?

The answer is epididymis.

Testicles contain seminiferous tubules, in which sperm is produced, and an epididymis, in which sperm matures. Mature sperm passes through the epididymis to the vas deferens. There, it combines with a liquid secretion from the seminal vesicles and continues through the prostate gland to the urethra. Sperm containing fluid is called semen and it is ejected through the urethra.