Dakuten used in non-standard ways (like with vowels)?

A dakuten in a manga means that the syllable sound is pronounced with emotion. It could be more loud, wild, low, or simply stranger than usual.

• あ゛ used to be commonly used in manga to express an exclamation — "aagh!" or something like that. I feel it has become less common (if not rare) these days. え゛、い゛、お゛、の゛ and so on are sometimes used in a similar way. You may see them in casual blogs and tweets, but never in formal documents.
• ヴ is commonly used to express the 'v' sound in loanwords (eg ヴァイオリン instead of バイオリン). It's usually in katakana, but in manga we sometimes see hiragana う゛ for various reasons.