[chopperman]

It’s ‘a’ history because the ‘h’ is hard.

[Verginius Rufus]

The "h" in "history" is hard because the syllable it is in has the stress.

My Webster's does not indicate any alternative to pronouncing the "h" in "historical" (where the "h" is in an unstressed syllable), but for the word "vehicle" it allows both pronunciations (in fact putting the alternative where the "h" is silent first)...likewise for "vehemence" (where the "h" is in an unstressed syllable).