If the boat came with the trailer the manufacturer will have that VIN. For instance... my Nitro is a TrailStar, although the dealer that was cosigning it for the Estate of the guy that owned it said it was an Ozark! So when I went to register and tag it the questioned it. Even though I had a photo of the VIN plate, AND the statement from BPS that it came with the boat, (they pull the hull ID the also have the trailer VIN).
I ended up having to get a special form filled out by the local PD. (Form 22b or something.) He came out and looked at it to verify it was what I said it was, then I went back to the tag office and they gave me the tag.
So yeah... if it came with the boat you should be able to get the VIN from the manufacturer. Just a thought.