To say that it sounded like a purely opportunistic theft and just 'one of those things' is really weak I know - but sometimes the truth does sound feeble, espescially when it sucks so much. Sorry.
There's little defence against being hoiked into a van, even if you'd have had a pager for your alarm it may not have helped much. Working in a team like that with a driver ready at the wheel gives little margin for stopping them.
Bottom line, no what if's or maybe's... there simply is no blame to be attached to either of you. It'll take time to realise this, and with the current situation as you've described with Vic's Mum - her reaction is naturally going to be to take things upon herself in the worse possible way.
Which is of course going to cause you to do the same.
Take it easy and concentrate on the positive things. A witness is a rareity, and one that has been on the ball enough to take down the van number is even more so. Here's hoping this little bit of good luck comes good and leads to arrests and hopefully a recovery.
Best wishes.