I think you'll find that in most cases in a 4-5-1 the guy upfront has to do significantly more than just tap the ball in and that a 4-4-2 is far far more suited to a goal hanging goal tap artist.
How many times did Drogba spear head a 4-5-1, did he only tap it in? No, he was on his own for much of a game, he needed to muscle the defenders, go long, drop deep, pick up and hold onto the ball till he gets support, get the ball and play others in as they race up the pitch and beyond the defence. How did Torres, a more notorious RVN type mostly finisher do in that same setup? How did he do with Gerrard working as a withdrawn striker but effectively right behind him?
Other 4-5-1 guys, Ibra at Barca got 15 assists... it wasn't because he played the tap in artist, like Drogba he won the ball, bullied defenders and held it up for others and played others in behind the defence constantly. RVP did more finishing and less holding it up largely as he was less strong, but dropping deep to get the ball and being an outball who could hold it and play in others was a huge part of what he did.
4-4-2 lends itself to a striker who holds it up and an out and out finisher(though works just as well with two strikers who can do either comfortably), 4-5-1 absolutely doesn't lend itself to a pure finisher, at all. Defoe, Torres, Hernandez.... none of whom shine under such a system.... who are all very much like RVN in their game, the best strikers who shine in such a system, Drogba, RVP, Ibra, Lorrente, Ade, Aguero, Tevez.
Some if not all of those players are great finishers, all of them have MUCH more to their game, and short or tall all of those guys do a huge amount of work for the team and spend loads of time holding it up and bringing others into the game.
RVN isn't your archtype 4-5-1 lead striker at all.