ヴ is usually used for the transcription of foreign words, like in the word グローヴ or even カーブ. So, you'll use the katakana ヴ. If I'm not mistaking, it's closer to the V sound than what ブ would normally do. Notice that you can still use ブ for some of those words; カーブ/カーヴ. The hiragana would be ゔ, but since the Japanese language doesn't really use that sound, it doesn't need it. Since katakana is mainly used to transcribe foreign words, ヴ would be used in order to more accurately represent the sound.
