Though both programs looks the same somewhat and appears to do the same things, there really are big differences between the two. Both records multiple tracks simultaneously based on the computer, sound cards, etc., but the similarity ends there.
For one, Acid could play loops and could loop. Changing/matching pitches, matching beats, etc are only one or two clicks away, while possible to do with Vegas, it was not born to do it and may be a little awkward.
Maybe the best way to differentiate the two is when you are mixing down your almost finished tracks. When you have to decide which tracks goes to what channel or channels and mixed with what tracks, to what effects to include, how many effects required, when you worry about your effects returns, stereo imaging, etc, etc.. I mean when you are doing mixing board stuff, that is where Vegas shines over Acid. But then, some of those things could also be done with Acid, you are right, but remember, I said SOME (not all), and they are not that intuitive like in Vegas. A few of the things you could do as simple tasks with Vegas, you would have to do a back flip with Acid, and vice versa. Multiple track recording - yes similar, but that is where it ends.
I hope that helps, but maybe, I just made it more confusing.
Download their demos and learn it first hand before you plunk down those greens coz they are not cheap.