There were two relics I picked up that REQUIRED me to use the Hang Glider to obtain. There's one on an island that I couldn't do the "hack jump" that I'd been using for all the rest of the islands. There was just nowhere to jump up to because of the sheer cliffs on the place. It stuck up about twenty feet high with no slope on it.
Then, there was the one in the ravine. There's a little shrine in the cliffside. I fell to my death ten or twelve times without ever ONCE seeing a way into this little nook. I saw no vines. I saw no alternate paths. I couldn't drop into it... I just went up the hill, grabbed the glider, then flew into the small nook for that Relic.
Beyond those two Relics, I didn't need a vehicle or a glider for any of the rest. So, if we're not counting Hang Gliders that were required for me to use to obtain a relic... I actually beat the game on Normal without using a single vehicle that wasn't forced on me.
And yes, BiggD... You can self heal. The problem with that though?
If you accidently pick up a "first aid kit", it actually gives you a syringe that you cannot drop or get rid of. At all. So if you try to heal after picking one of those up, you'll use the syringe and forfeit the achievement.
"It's worth remembering that all reviews are subjective personal opinions and if you personally enjoy the game, then they shouldn't get to you. Unless there's a dispicable nagging little doubt in the back of your mind that maybe you're not having as much fun as you've convinced yourself you're having. And won't go away no matter how many times you try to slap it away with weak excuses."