I'm thinking of leaving the job when my boss is in desparete need of me
Why would you wait to quit until your boss needs you.
I don't like that job and I am doing too much for little pay.
Try to negotiate with your boss about a higher wage. I am sure he will understand from all the work you are doing at work.
Nah. I'm going to ask for my vacation and then quit when my boss needs me the most.
I know it sounds like you are teaching them a lesson, but you are only going to burn a bridge you may need in the future. If you want to quit, fine, but give two weeks notice.