Anti-gravity vehicles?

Depending on which scientist you ask, antigravity is either impossible or impractical (as in, the theory seems to allow it, but it requires ingredients that might not exist or that might be impossible to create). As such, if you have working antigravity devices in your story, they work however you want them to work. Just make sure that whatever rules you make up, you apply them consistently.

Saying that, it seems obvious that an antigravity device for use in a vehicle would need a control to turn it on and off, or to adjust its strength. Otherwise it would stay at the same height above the ground (where it balanced out Earth's gravity) or would fly off into space. If the vehicle is wrecked and lying on the ground, its antigravity mustn't be working. So even if the passengers do float around when it's flying, that wouldn't happen if they enter it when it's on the ground.

I don't think you understand gravity. If you're in a plane and that plane starts a decent at a high rate of speed, then you will experience weightlessness or anti-gravity.

In fact, if you build a tower straight from earth to the international space station and then stood on top of that tower, you'd just stand there. You're not going to float away or anything. Now, it would be a different story if you fell off that tower. Then you would be in a constant state of acceleration as you orbit the earth and just like with the plane decent at a high rate of speed, experience weightlessness.

dd