this is kinda situational.
Renekton excels at early and mid game but falls of in late game.
Vlad is the opposite he's weak early, strong in mid and godlike in late game if played properly.
Both are good but vlad forces you to play passive for the first 6 levels, while renekton goes for total lane dominance from level 2 onward.
Kinda depends on what you want: Vlad-> a tanky ap/bruiser with high sustain and damage and no cc, with a slow base ms and a mediocre escape. can also be played mid.
Renekton:-> a tanky ad/bruiser with strong sustain, a cc and a dash as well as an ulti that gives hp. Can push really fast and has a decent base ms.
If you play top only then go for renekton first, since he counters way more champions on top then vlad does.