I suggest ...
just simply add a "Rename" button in the window of viewing a vehicle info. ;)
because some of the vehicles have a very long name
it takes much time to delete the original name
and type a new one :P
Personally, I would rather have the ability to delete highlighted text (selected with mouse or cursor keys), as well as cut/copy/paste capabilities.
However, if a button to clear the name was easier... it would be better than what we have now. :)
But you can change name of vehicle without such button - in depot. All you need to do that is focus the first line (where is name of vehicle) and write.
that is implemented in the nightlies ...