Best thing you can do is have as many means of security as possible, none of them will stop a theft but having multiple layers will likely deter them.
Van alarms usually come as standard but a thief who is good wont care, no one takes any notice of alarms going off.
Deadlocks will just mean they ruin your doors to get in.
Best solution for theft is probably a pedal box and a good steering wheel disc lock, throw on a wheel lock as well if you like, more security measures the better. Thieves are lazy if they see all that lot on they will just go for an easier target.
My van is behind locked gates, it has a steering wheel lock and i have a floor security bollard on the drive. Doesnt mean it isnt stealable but why bother when theres thousands of less secure vans to target.