Romans only used the letter V to represent both sounds, so there is no difference in sound no matter which you use. U was originally just a variant, but eventually it adopted the vowel role and V kept the consonants, similarly to what happened with I and J. Technically, "malum diluere" could be written "malvm dilvere", but again, a modern reader would not know how to pronounce "malvm" without some explanation. "Diluere" may not seem that unusual to us who are used to Latin (somewhat at least), but I wouldn't want those barbarians mangling it into something sounding like "dill-yoo-uh-ray"
though there is only so much you can expect from those ignorant fools
flebile nescio quid queritur lyra, flebile lingua murmurat exanimis, respondent flebile ripae