i would be tempted to go down the physical barrier route, i wouldn't chain the van to anything because they might not notice that untill they've got it running and have started to drive away and could just leave you with a broken mess. sink a bollard into your drive, a big one and not a fold down one, something like this

a wheel clamp is also a good visual deterrant, if they can see it from the road, and again only the good quality ones, i've got one of the cheap ones on my trailer, couldn't find the key last week so i jumped on it and it fell off!
An alarm is still a good idea, it will alert you to something happening even if no one else pays attention, it won't stop them but it might slow them down.
have you decided what sort of van you are going for? sticking with t4 or something else?