First, as long as he's outside and you're inside, you have both a tactical and safety advantage. Leaving the van is a poor initial tactical decision. The odds are good that he doesn't know that the van is occupied, and as long as there's the van between you and him, he doesn't know where you are exactly and he can't touch you. If you leave the van, he knows EXACTLY where you are and he CAN go hands-on with you.
Since you have those advantages, deterrents in that situation are your best first move. The horn can be VERY loud when you're standing beside the engine compartment... and does several things: it'll make the suspect jump, telegraphs loudly that there's someone inside, and third makes noise that others may wake to and at least look at. Since car prowlers are primarily interested in what they can steal, not being in a confrontation, the odds are very good that all you'll see are feet running away. That will also force the crook to play his hand if he's more interested in an attack than being a car burglar.
If he sticks around and continues to try to break in, yell that you're armed and he's in a world of hurt if he gets through the door... and when he DOES get through the door, do what you need to do to keep yourself safe.
