Why do you put an after a vowel?

It seems like you're asking about the use of the apostrophe (') after a vowel. However, there's no general rule that dictates putting an apostrophe after a vowel. Apostrophes have specific uses in English grammar, and they aren't typically placed after vowels just because they are vowels.

Can you give me an example of what you're referring to? I might be able to understand your question better with more context.

For example, are you thinking about:

* Contractions: Like "don't" (do not) or "can't" (cannot). These use an apostrophe to show missing letters.

* Possessives: Like "the cat's tail" or "the girl's bike." These use an apostrophe to show ownership.

* Plural of letters: Like "a's" or "b's." This is a less common use but still valid.

Once I know what kind of context you're interested in, I can give you a more specific answer.