Hello,
Nope, if you are willingly taking negative Traits and are spending XP on vehicle Traits, then you also would gain XP from picking negative Vehicle Traits....
...of course, per p. 107, you can only use up to 10% of your maximum starting XP in negative Traits. If you got them from your ride, obviously, it'll affect your character less, but you still have to live with them... Until that vehicle you poured XP into gets destroyed.
Remember, the rulebook isn't there to straitjacket your playing; it's the GM's job to ensure game balance.
Hopefully, that helps.
Thank you,
- Herbert Beas
BattleTech
Catalyst Game Labs