do they max out real fast or is this just a big coincidence? I just checked them for the 1st time and for normal it says I have 999 covenant kills and 99 vehicle kills. thats from playing through the game once. so its rather pathetic if that truly is where is maxes out at.
Unlocking the bronze commendation during campaign requires a total of 200 enemy vehicles destroyed. I'm at 129 at the moment, so it definitely doesn't cap at 99. I have less covenant kills than you in campaign (only 800-ish), so I can't vouch for that side.
I have about 2k headshots in campaign and on the Silver tier for the Pinpoint Commendation, so there's loads of coveys to kill... but that's on Legendary.
No, they never max out. They are simple variables, so if you go get 1 more kill you will have 1000, or if you turn around and get 10000000 you will have 10000999 kills.