[name_m]Vladi[/name_m] and [name_m]Vova[/name_m] are nice! My first idea was [name_m]Lad[/name_m]/[name_m]Laddy[/name_m], although I’m not sure how much he’d want to be that growing up? I think Miro is also a really great nn, a bit of a stretch, but still lovely. (Or Mimo? Apparently Mimo’s a traditional Spanish nn for [name_m]Maximo[/name_m], but I think it could work for [name_m]Vladimir[/name_m], too…)
I have to say I think Miro is my favorite, though. 
Good luck!