401(k) questions.....

When am I considered 50/100% vested? (I can't find any information on this.)

Also, say I don't contribute anything for the first six months and then I start contributing 15% per month. Can I play catch up with the company matching? Or in order to get the 50% of 7 up to 3.5% do I have to contribute every month? ( I have to say my knowledge is very limited on 401k matching so please dumb down your responses for me.)