[dsraven]

vin plate should be rivetted on, noy spot welded. special rivets too. if it is spot welded it has possibly been off before. I suggest to ask at the local DMV before you go further.
here in Alberta Canada, it is illegal to tamper with the vin plate and on my project, which took 2 cabs, the vin plates were taken by the inspector and a new vin plate was issued. they needed to see the bills of sale and pics of the 2 vehicles on site. this is to ensure there were no stolen parts etc. not a cheap undertaking. check your local laws for sure.
have you got a pic of the spot welded areas? just curious of what they look like. tape over the vin before you take the pics and post them.
otherwise, a couple of layers of heavy duty duct tape and talk to the blaster. maybe even label the area around it with "do not blast this plate" just as a reminder, sometimes the guy you talk to isn't the guy doing the blasting.
it goes without saying that you need to find a blaster familiar with blasting automotive sheet metal. otherwise you will have nice clean parts that are warped out of shape. a good blaster for automobile bodies will know about the vin plate issues.