Your second example doesn't work because that isn't how you call a function, you didn't say if it was true or false.
Your if statements don't make sense either, I don't actually understand what you're trying to do with that, one = assigns a value to a variable, it doesn't check if it's 'equal to'
On the client side, "playerEnterVehicle" doesn't have a player parameter so that's why your event doesn't work.
Client-side
mp.events.add("playerEnterVehicle", (vehicle, seat) => {
vehicle.setInvincible(false);
});