The plate being on the dash, versus the A-pillar is up to the manufacturer. They are in the same general location, where they are plainly visible from the outside of the vehicle. That plate is the "public VIN", and there are hidden VIN's stashed all over the vehicle, especially on newer cars/trucks, intended to help identify theft. An 80's Ram will likely have some hidden VIN's but not as many as a newer one. It's something that has evolved to aid in recovery of stolen cars/trucks.
The next step I'd recommend for you, is to have an inspection done on the truck to verify it's not stolen. The VIN plate shouldn't be easy to take off, so unless you cut/ground the rivets out, it would lead me to believe someone else already did it. I don't want to freak you out, so I want to clarify that some people get a little over zealous when restoring/repairing older vehicles, and they'll take the VIN plate off without malicious intent (case in point: for whatever reason you took it off). It's also possible, though unlikely, that since it's a 30 year old truck, the rivets and/or plate have been affected by age. While that plate having been removed is an indicator of possible theft, it's not concrete evidence of it.
It may not be stolen, but you don't want to be blindsided by it if it is. A salvage inspection should be able to clear that up for you. If it is, and you were an innocent buyer you're not in any trouble (at least we don't charge them with anything). If you're lucky the title shows the prior owners information, or maybe you still have it. If you bought it off Craigslist, Ebay, ect, the site may have an archive that is accessible to law enforcement. If it is a false VIN, and you have a title with that same VIN on it, the person who sold you the truck is also looking at Title Fraud.
If it's not stolen (fingers crossed) you'll have peace of mind knowing that there's no legal surprises waiting for you.
To answer the other question though. If it's stolen, the court would require the fraudulent seller to reimburse you damages. That could be for the purchase price, time wasted on the ordeal, any money you've already put into the truck's repairs, title/registration fees, etc.