The most common and most likely answer to this crossword clue is the 6 letter word MOPEDS
I've seen this crossword clue in The Daily Telegraph
M
O
P
E
D
S
More clues leading to the result MOPEDS
Buddy explains!

The straight part
We know that "vehicles" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case vehicles = mopeds.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does MOPEDS mean?
MOPEDS noun- A lightweight, two-wheeled vehicle equipped with a small motor and pedals, designed to go no faster than some specified speed limit.