This is a relatively recent addition tokana usage to represent the non-native soundvu. As such, this is only found in borrowed words. Many speakers, especially older people, may pronounce this more asbu,[bɯ̟ᵝ].
The soundvu appears almost exclusively in borrowed words, and is therefore normally written inkatakana asヴ(vu). This hiragana character is used much more rarely, and exists in computer encodings primarily for correspondence with the katakana form.