usage - When using "an" before a vowel sounds wrong

Consider the following sentence:
"This is a one-time deal" sounds right
"This is an one-time deal" sounds wrong

"One" is pronounced the same as "won", which wouldn't require an "an".

Is it proper/required to use the 'an' before a vowel rule when it just sounds wrong?

Answer

There is no rule that says you must use an before a vowel, only before a word that begins with a vowel sound and takes the indefinite article. University begins with a vowel but not a vowel sound, so it's always a university. The same is true for a one-time deal.