Here's why:
* Prefix: A prefix is a word part added to the beginning of a word to change its meaning. There's no part of "vague" that can be separated and identified as a prefix.
* Suffix: A suffix is a word part added to the end of a word to change its meaning or grammatical function. Again, there's no part of "vague" that can be separated and identified as a suffix.
"Vague" is a standalone word with its own meaning.