Scenalyzer has some nice things about it like optical scene detection. It can be really handy if capturing from analog sources, or on tapes without date/time on them.
Scenalyzer can also capture all 4 tracks of 12-bit audio at once. In most editing programs that would take 2 passes. 12-bit audio is lower quality and 16-bit audio however.
Scenalyzer is nice, although you may not really need it.
Vegas can't capture the second stereo channel at all.
Optical scene detection will detect changes in scenes based on changes in the actual video instead of requiring jumps in the date/time code. It can be useful when wanting to split scenes and the clock in the camera was not set, for example.
Scenalyzer can also do a really nifty "contact sheet" of an entire miniDV tape. It runs the camcorder at "fast play" speed (10x) while building this, so it only takes a few minutes. It makes for a decent way of keeping track just what's on the tape.
I don't know if VidCap can do this, but I suspect it can't.