v.154 – Sword Art Online Update Notes

It says that if you die or DC then all your progress disappears 'as if it was a dream'. It seems to indicate that all gains you get in there are rolled back. I'll test that on some mule.

So if I hear you correctly, I can fix the vehicles and stay in the area? (As in, it's a separate thing to fix them than it is to use them.)

Yes. After you fix one, you get to choose whether to use it to escape the island, or continue to explore. You can also "End Chat" at that point if you'd rather. The vehicle will stay fixed, so you can use it later if you want to.