Hi and welcome to the forum! I'm sorry to say I've had vehicles I cared about stolen and vandalized too, so I know the mental aspect of what you must be feeling. There are few things I hate more than thieves and vandals. I'm glad to hear he wasn't successful in getting it started. Hopefully he gets caught soon.....even better if it's by somebody that hurts him bad.
As for advice, the 1st thing I should say is I can only offer general advice as your van is quite different than anything sold here in the American market. Toyota never sold diesel Previa's here, and I've never seen one, so I'm not sure where the similarities end or begin. That being said it stands to reason that the problem must be something he did. Since he was so destructive, it will be hard to rule anything out.
You didn't say if it was an automatic or a manual transmission. If it's an automatic, my 1st thought would be to check the neutral safety switch and/or the linkage that attaches the shifter to the transmission. If he bent that linkage the safety switch may not be in the correct position (to allow the starter to run). If it's a manual transmission, then you should check the clutch safety switch and the wires to and from it. Other than that there's not much I can offer, other than suggesting you get a Factory service manual for your specific vehicle so you can use the electrical diagrams to help you troubleshoot the circuit. Good luck! Tim