1. Dashboard--Dodge Sprinter VIN
As AutoStart stated, this VIN is on the dashboard, near the bottom of the driver side A pillar (visible through windshield).
Mine is a sticker, stuck on crookedly, on a tiny plate.
2. Driver Side Seat
Open the driver door and look at seat pedestal side facing door (take velcroed seat pedestal cover off). Don't remember all the info there, but included tire pressure.
3. Sliding Door Rear Jamb
This sticker is from Westfalia and specifies the Westfalia model and which number made.
4. Inside Closet Door
On the inside of the closet door, there is a sticker labeled "Motorhome Weight Information" or something like that. Sticker has VIN number at top, and the various weights of different items.
If I remember correctly, the VIN number on label has a bunch of extra digits/ letters in front of the VIN number on dashboard.
The first time I had to get a smog check in California, the inspector was looking all over vehicle for that info until I showed him where it was. The vehicle is registered as an Airstream and not a Dodge, and at that time, the smog inspection was visual only as the smog computer didn't have all the info. Later smog inspections, inspectors never asked about weight or where weight info was.
More Info: Nice person here will run your VIN and give you all details of your build. Some Westy owner did that and found deleted seats (Westfalia installed own ISRI seats), etc.
https://sprinter-source.com/forum/showthread.php?t=40575