Should VAC banned users still win games / register on this website ?

You're pretty far on this list.

I think one offense, no. There are many reported cases of the system screwing up, and Valve cannot reverse the ban. If 2 VAC bans, then they definitely shouldn't be able to win a game.

I'm not sure whether Support would sanction a re-roll for this. I suspect not, but you could always give it a try.

Ultimately, if you really have an issue with the winner, you remain at liberty to take the "not received" strike and not give them the game, whether or not Support approve a re-roll.

Since they can't run a program to cheat for wins here, it shouldn't matter a jot. As long as the game is enjoyed by the winner, that is all that should matter.

What does cheating in a game have to do with wanting to win and/or give a game away to someone on this site ?

You may put that condition in private or groups giveaways, after asking the mods for permission. In public giveaways? Nope.

Anyways, one of the older SG's users with a huge contributor value had a VAC ban in his account. Should he be banned from winning anything?

Honestly I couldnt care less if someone is VAC banned on SG. It's a little different when I'm trading (a matter of trust), but here it doesnt matter even a little to me.