Potentially not delivered. That could mean that it was refused by a picky customer, refused by someone who noticed the seats were missing, not purchased because their financing fell through, not purchased because they realized they can't handle the latest interest rate on their financing, not purchased because they want to wait for the tax credit, etc. Just go to your delivery and see if the vehicle is what you want.
If you're thinking that you don't want a vehicle dirtied by somebody else's backside, remember that it has probably been driven by a half dozen people in the process of getting it to you.
A VIN out of the blue usually means a deal fell through (see above) and Tesla is looking for a matching order at that delivery site. It's a new vehicle. Just look it over as you would a vehicle with the absolute latest VIN number. Mine wasn't ever assigned to anyone else and it had a dent. Others have taken delivery of pristine vehicles that were reassigned.
Folks, please put your order information in your forum signature so we can know where you're coming from in your comments. The option of specifying a forum signature will be available to you shortly after you complete your 5th post.