"It is not in law a vehicle."
"A vehicle is defined as: any conveyance in or by which people or objects are transported, especially one fitted with wheels"
If I asked you to fetch me a vehicle to get from A to B, I'd be well annoyed if you came back with a Caravan!!
We are talking legalities as concerns a Trailer which I'll grant you is a vehicle in the general sense, but, in this discussion as you yourself point out, it's sub-divided and defined as a Trailer.
"A vehicle is defined as: any conveyance in or by which people or objects are transported, especially one fitted with wheels"
If I asked you to fetch me a vehicle to get from A to B, I'd be well annoyed if you came back with a Caravan!!
We are talking legalities as concerns a Trailer which I'll grant you is a vehicle in the general sense, but, in this discussion as you yourself point out, it's sub-divided and defined as a Trailer.