"Oh, there you are!"
Translation:Tiens, vous voilà !
I think it's tricky because vous is both a subject pronoun and an object pronoun, which might make you think that you could then use tu for the singular/familiar. So wouldn't it then be a matter of using the corresponding object pronoun, te? "Tiens, te voilà !"?
[Update: I just tried that, and it doesn't work.]
'Tiens!'/'Tenez!' could be translated to 'here you are' or other exclamation aimed to attract attention to something; voilà and voici are used with me/te/le/la/vous/nous. I'm no expert tho! That's just what I've found.