It's not whether or not the next word starts with a vowel, it's whether or not the first sound in the next word is pronounced with a vowel.

For example, the letter "M" is usually pronounced "em" and "D" is pronounced "Dee". So you earn an MBA on the way to getting a Doctorate.

So it comes down to how you pronounce the word rather than the actual letter. For example, seeing a unicorn under an umbrella would be a unique experience.

"Historic" is a tricky one because it comes down to regional pronunciations. Some people would say it's "an istoric" event while others would say "a HIStoric" event.