Well SB also work on FreeDOS (right?), still 6.x will always be better.
Like Virtual PC, it emulates a perfectly working SB16 most of the time... but when I try to play Tomb Raider it doesn't work perfectly. Not to talk about other millions of issues I find if I try to play other DOS games in there--the same as in a real DR-DOS I'd bet. (Of course DOSBox beats any other solution for 99.9% of the games, real DOS 6.x included, but you already know that.)
I haven't checked myself, but according to the
Wikipedia article, even though no 64-bits Windows (XP included) includes a VDM, the 32-bits version of Vista would have it just like XP. (Otherwise the article must be edited.)