You are implying that a vasectomy would lower LH/FSH, but on TRT, LH/FSH-->zero. You are thinking of a guy with a vasectomy starting TRT, or vice versa? I do not see how a vasectomy is a factor.
On TRT, if you do not use hCG to support the testes, they will shrink, possibly dull ache 24x7 and scrotum pulls up tight. A few guys, quite uncommon, do not experience that, suggesting that there are some that do not need LH receptor stimulation which in turn suggested genetic variation or differences in gene expression.