I’m afraid you have to do this manually. You can put someting like this in your single-vehicles.php
file:
if ( ... ) { // check if it's in used category/term
get_template_part('part-single-vehicles-used');
} else {
get_template_part('part-single-vehicles-new');
}
And then put new car template into part-single-vehicles-new.php
file and used car template into part-single-vehicles-used.php
.