The best way to make requests is to have MS put them on your reservation. There have been several posts here that indicate faxing does no good and in fact, request faxes are not even looked at. There are just too many of them. I have read that VWL isn't accepting them at all.
To the best of my knowledge, rooms are assigned approximately 3 days in advance.
Ask MS to list your requests in the order of importance to you. We generally list N/S first as that is the only one that really matters to us. It's best to make your requests as general as possible. The more requests, the less likely it is that they will be granted.
Enjoy your trip!