What I don't understand is how can vampires with +1 stealth and +1
intercept get those bonuses when they're not needed. Or, how someone
with a Bowl of Convergence gets the intercept before it's need. Can
someone explain this to me?
He doesn't add +1 intercept; he *has* +1 intercept. Those effects are
"on" when the action starts. The rule prohibits adding more when it
isn't needed -- it doesn't prevent existing stealth/intercept from
coming along.
See also hunting's default +1 stealth.