Question: I just grilled up a steak marinated in vinegar, rosemary, and olive oil. Problem is I made too much. What's the proper way to freeze, store, and subsequently reheat this steak to get it as close to as delicious and tender as it now?

Answer: 
Generally speaking, you don't.

If you really must freeze steak, freeze it before cooking and *slowly* defrost it in a collander overnight.

Re: your current problem - I would wrap in tinfoil and heat in the oven. A pan may overcook it.
First wrap in foil. You don't want plastic anywhere near your steak if you can help it to avoid that plastic frozen dinner taste. Then wrap airtight in plasric wrap - not plastic bag.