The safest thing to do is remove the grounds and plugs for all the electronic stuff, especially the van PCM, BCM, ABS, etc, but also any chargers or controllers you have in place. It is a huge job on a lot of vehicles.
The couple welders I have talked to have said the what kind of welder being used makes a big difference and also having the frame grounded to earth can help.
TIG is said to be the worst because of the high frequency AC start it uses on most decent setups, AC stick welding next, then DC stick and MIG wire welders.
I have seen a lot of shops using a wire welder on vehicles with no precautions, so it can't be a really big deal, but there probably always is some risk involved. I have used wire on a vehicle a couple of times without any problems, both with and without the earth ground. (fire risk scares me more as you never really know what is on the other side of frame tube or panel) If it were big stuff like a frame repair (I was welding moulding or panel repair pull holes) I think I would take more precautions, but I really don't know if the higher power needed is worse than pulses or not.