That's kind of an interesting idea. So say an assault `Mech would have a group of quirks that adds up to -1, so you could purchase that assault `Mech at +7 instead of the normal +8?
Pretty much, yeah. You could also apply it to the Custom vehicle trait, so that same -1 quirk could get you choice of a stock medium mech at only +1 TP in Custom Vehicle (though to get both of those choices would require -2 TP of quirks).
It can go the other way too, if you have enough to own an assault mech, but only want a heavy, you can get a few positive quirks on your mech. Similarly, if you somehow have Custom Vehicle at 6 TP, but are restricted from clan mechs you could instead get an IS mech with +2 TP of quirks.
I suppose it could also let you transfer TP between Vehicle and Custom vehicle; rather than getting a random heavy, you can choose a medium, but you have to take a positive trait which would raise the Vehicle TP requirement and a negative trait to lower the custom vehicle requirement.
You know, I wonder if it might not be possible to combine the Quirks and the Vehicle trait.
That's essentially what this is. Vehicle Price plus or minus any quirks. THe quirk doesn't exist on the character sheet, it just makes the mech cost more or less TP.
The bit that keeps getting me is the disconnect between vehicle and custom vehicle.
I suppose if you want to combine those you could have the Custom Vehicle Trait be a modifier on the Vehicle trait along with the +2 to own a mech (So you can have a random medium mech assigned at +4 Vehicle; at +6 you can either own a random medium or choose a medium that is assigned to you and at +8 you can choose a medium mech to own ). Modify the requirements for quirks as well.