"Of course the original designer should be credited, but in TWDA the player goes before (and the designer should be mentioned in the Description field). No matter how good deck you play, the player himself plays a significant part in VTES."I totally agree with the above statement. Most players (or maybe even some non-VtES players) can build a decent VtES decks that is capable of winning a tournament (large or small). But playing a deck is an entirely different matter, since it's not just about the mechanics or the theoretically best play in a given situation, but other things like the overall situation at the table and table talk between players have to be considered, too.
I even daresay, that the truly good players are able to win with really not overly good decks (to say the least). Examples for this are (in my opinion) some decks played by the American players Matt Morgan and Jay Kristoff, e.g. the Fuzzface deck by Matt or the Nosferatu Toolbox w/ Trophies by Jay.
So, while the original creator of the deck surely deserves some credits, the more significant part of making a deck a tournament winning one is done by the actual player.