I repaired a couple of vehicles the other day (I typically equip mines) and didn't get any points for it. I have no repairs listed on my BattleLog either, but I recall the vehicle being fixed.
The Battlefield Wikia has this to say on the subject of repair:
Using a wrench or repair tool to fix a vehicle, device, or strategic object belonging to the team.
For vehicles, another teammate must be occupying the vehicle in order to earn repair points. It is common courtesy for a player to enter a vehicle being repaired solely to allow the engineer to earn repair points, and to give back control when the repairs are done.
I also see several threads that say if you're in the vehicle when repairing it (especially aerial vehicles) you won't get credit, including a question on this very site:
I don't believe that you need to repair any particular percentage of the damage done to a vehicle, there is/was a glitch in the game about repairing EOD bots that are being damaged by teammates, and you can see in the video on this page, this guy's getting tons of repair points for relatively minor repairs.
Doing a repair gives you +20 points, and a Squad repair counts for +30.