Anti theft system

Hello Welcome to 2CarPros. Sounds like you've had a bad few days.
First step, file a police report, this way if your van suddenly turns up missing they have an idea who to look for and they might even get the fobs back if they can.

Next, the starting and immediate shut down says the anti-theft immobilizer is functioning.
The fact that you can turn the ignition is a good thing as it means a lot less work to change the ignition lock cylinder out. However, even changing the locks will not make it secure if the fobs are used. The steering wouldn't unlock without the key but anything else would still work.
Personally I would opt to have the ignition lock changed then the door lock(s) keyed to match the new keys. For the fobs there are a couple options. One is get new ones and have those programmed in along with the keys and have all other fobs erased from the system. That way if anyone tried to use the old fobs they wouldn't work, and if you get them back you have spare parts.
The other option is to just have the old fobs removed from the system and use only the keys.

All of the above options will require a factory level scan tool or one of the programmers like an automotive locksmith uses to clean the system of the old information and install the new parts. Changing the ignition cylinder itself isn't real hard but the rest of the process requires the tools to do the programming or you will have the nice new keys that will start the engine and it will shut off instantly. A dealer could do all this but usually a locksmith is cheaper.