What about saying "a 0day" which I read as "a zero day <attack>".
This same thing happens with "MVP" I think. "An MVP" sounds good, but "An Minimal Viable Product" doesn't. So I use a/an depending on whether using the abbreviation or not, but I have no idea if there's a rule for this or not.
As far as I'm aware, the rule is if the first syllable starts with a vowel, phonetically.
In your example "MVP", the first syllable starts with an "em", so a vowel. In "Minimal Viable Product" the first syllable starts with the consonant sound.
I had a major pain with this rule when dealing with auto-generation of some API documentation. It ended up just being an issue of detecting/guessing if the text was an abbreviation or some weird CamelCase thing.
This same thing happens with "MVP" I think. "An MVP" sounds good, but "An Minimal Viable Product" doesn't. So I use a/an depending on whether using the abbreviation or not, but I have no idea if there's a rule for this or not.