You missed Belarusian, where is it ў, pronounced /w/. This is significant as it is the common intermediate between /v/ and /u/.
But I'm not sure how to answer you, because I'm not sure what your question means. Certainly /u/, /w/, and /v/ can replace each other between languages, or even within a language in different phonological contexts.
For example, in Classical Latin times, there was no /v/, and /u/ and /w/ could show up in the same morphological context but different phonlological ones. Consider for example the perfect suffix:
1st conjugation: stem: -a-; perfect -avi, eg amāvī /ama:wi:/
2nd conjugation: stem: -e-; perfect -evi, eg dēlēvī /de:le:wi:/
3rd conjugation: consonant stems; perfect (for some verbs) -ui, eg posuī /posui:/
So the /u/ and /w/ may have been felt to be "the same sound"; in any case, there were no problems using the same letter for them.
Later, when the consonantal pronunciation became /v/ in most varieties of Romance, the same letter continued to be used for both /u/ and /v/ until comparatively recently, when two forms of the letter started to be consistently distinguished. English required a way to write /w/ as well, so a third letter was created from by doubling the existing one.
The fact that only Serbian and Croatian among Slavonic languages have vocalised the (Edit:) consonant in the preposition has nothing to do with the way it is written.