They certainly could be! In Classical times, there was no distinction between the letters V and U (or between I and J). The name "Venus" would be written in inscriptions as VENVS.
However, there was a difference in pronunciation: sometimes the letter V represented /w/, and sometimes /u/. And those pronunciations stayed distinct in the Romance languages, leading to the difference between modern "u" and "v".
So modern editors typically show this distinction by writing the consonant as "V" and the vowel as "U", and similarly for the consonant "J" and the vowel "I" (though for some reason editors are now going back to using "I" for both: "adiuvo" instead of my preferred "adjuvo").
One exception to the rule, in modern editions, is that "U" is typically used after "Q" (even though there was no /u/ vowel there). So we see "equus" instead of the expected "eqvus" (compare "flavus").