Not sure that I know what you mean by 'van' Desmond.
If you mean something like a Ford Transit then I think you will struggle to find one, but don't think it would be very well suited to 'sightseeing' either.
There are mini buses around for certain, but I do not know anywhere in Udon that hires them.
A 4 door pickup might manage your needs or even a Fortuner. Lek Car rental would be my recommendation to hire one of those. Their site is here..
http://www.lekcarrentaludonthani.com/I particularly think they are worth you talking to because they offer a fair range of different vehicles & good comprehensive insurance cover - something that you need to take care to ensure you get with some hire companies.
They also offer the option of drivers.