Would having a vasectomy in the past prevent a future bacterial std from causing epididymitis, since the tubes connecting are cut, preventing bacteria from reaching the epidimis? can epididymitis be the only symptom and start 40 days post exposure?

Vasectomy does not prevent epididymitis. Infection can extend along tissues, lymphatic vessels, etc -- not only through the vas deferens (sperm tubes). Epididymitis due to STD (gonorrhea, chlamydia) usually has obvious urethral symptoms like discharge and/or painful urination. See a doctor if you have severe pain, usually involving only one testicle.

Though the chances of retrograde infection are reduced- epididymitis can also occur thru blood. STDs can cause different symptoms from each other and time of exposure depends on many things like virulence of organism and a persons immune system. Good question