What to do when you can't get financing for a new van after your old van was just totaled by someone else's negligence?

There isn't much you can do, except shop for another used van with whatever you get from the person's insurance company.  The law doesn't require the negligent person to put you into a new vehicle, they are only required to pay the reasonable cost of repair or the fair market value, whichever is less.

I know that you probably won't be able to actually buy something as good as your old van, with the money, because it never works out that way.  But that's a problem that the law won't help you with.