This seems to be the basic shape of Nightow's male characters: slim waist, long legs, long arms. The first one is "mortal" proportions, eight heads tall. The second one is "hero" height, nine heads, but if you take a look and compare the two, the extra head height is not distributed equally along the entire figure. The arms are the same length as the mortal proportions, but the legs are much much longer.
I am pretty sure sometimes Nightow draws his characters almost 10 heads tall... Every once in a while in the manga, Nightow will illustrate a kind of idealized version of Vash, like, the mythical version. One of those pictures has IMPOSSIBLY LONG LEGS. (Still looks cool, though...)
edit: FOUND IT. [link]
There's no doubt about it, folks. The extra height is in the legs. Even the character design for Vash on the anime is crazy disproportionate. If you look at the model sheet for the anime, you can see that out of 9 heads, 5 of them are LEG. If you compare this to the mortal proportions, it's only 4 heads tall. If you look at NORMAL hero proportions, the legs are still only 4.5 heads tall.
Vash character design probably belongs to either Yasuhiro Nightow or Madhouse Studio.