The H in "history" is sounded because the first syllable is accented. But in "historic" the accent is on the second syllable, so both "an historic" and "a historic" have advocates.
Similarly with vehicle and vehicular. In "vehicular" the second syllable is accented so the H is pronounced, but in "vehicle" the accent is on the first syllable and some people would not pronounce the H in that word (VEE-ick-'l).
I think that’s exactly right, but somehow to me using “an” sounds snooty even if the second syllable is accented and the “h” is silent making it grammatically correct. Doing it the other way is also grammatically correct but sounds more natural to my ears.