I want my vehicle to "jump" when the player presses space. Would I do it with Keyboard#isKeyDown() or make a new key bind for the space key?

I'd say make a KeyHandler and add a vehicle jump key (So the player can change it if they so choose) and then, handle the jumping. I'm pretty sure the horse does kind of what you're wanting to do so maybe check out that.

Well I did that, but then Minecraft won't jump because the vehicle jump key is registered for space instead of player jump. Is there a way around this?

Check if the player is riding an entity and, if it is your vehicle. Then: disable space for jump and enable it for the vehicle the player is riding. When the player is riding a horse, the space bar contributes to the horse the player is riding, otherwise, the spacebar is for the player's jumping.

Check where

Why not just use the regular vanilla jump key? You can handle that in a KeyInputEvent just like you would a custom key binding, with the advantage of being very intuitive for players (jump key jumps for everything) and still customizable.

 

Another alternative you may consider is that you could simply let the player jump as normal while riding your vehicle, and then have the vehicle y motion match the player's, then you wouldn't need any key handling at all. I doubt that's how horses / minecarts work, but depending on what your vehicle is (e.g. a pogo-stick ), it could be an option.

You can use Minecraft.getMinecraft().gameSettings.keyBindJump.getIsKeyPressed()