What I'm gonna try and find out is how different they really are.
We need a proper DVD for this. One with lots of colours, lots of detail... and a proper movie.
I've nominated Twins Effect 2. Yay !
Note: I can't grab the exact frame with VLC (when you click on "pause", it takes 3 seconds to stop
Here goes:
PowerDVD 6:

VLC 0.84:

MPC 6.4.9.0:

VirtualDub 1.6.15:

Next screencap will be posted later.
But so far they all look EXACTLY the same (except the PowerDVD cap which is very slightly darker than the others).
PowerDVD:

VLC:

MPC:

VirtualDub:

Aw for fuck's sake !
I'm wasting my time here, they're all the same.
MPC = VirtualDub.
PowerDVD = VLC (both are slightly darker).
Conclusion: take your screencaps with whatever you want, they'll be fine.