We just got a 2BR shortstay for the Grande Vista over Christmas holiday.
Can we can request a certain location?
If so, what's the best building/unit to ask for?
You can place a request but not likely this far out. Your unit and building options are going to depend on what unit type you have. You can see this in your history in II. Once you report your unit type others will be able to give building advice. There have been quite a few threads on this so you might find what you need with a search.
An II short stay is actually just a normal 7 day Marriott reservation unless things have drastically changed since I last messed with short stays. What will happen is when you look up the reservation number on marriott.com you will see that it is for 7 nights. The resort supposedly gets info from II on which nights belong to you. The other nights technically still belong to II but to my knowledge they have never acted on them. For example the remaining nights do not stay in the short stay booking for someone else to see.
I have experienced various results with short stays in Marriotts. I did a 4 night short stay where I had the first 4 days and Marriott checked me through the full 7 nights. I have done several 6 day back end short stays where Marriott checked me in a day early. At the same time I had a 5 day back end concurrent short stay but they didn't check me in until the day I was supposed to check-in (even though the unit went empty).
If you are on the front end of the 7 days then you will likely receive the added benefit of not having to check-out at 10AM.
If things have changed and your marriott.com reservation shows the short stay dates please let us know. It would be nice if II eventually fixes this and finishes programming the short stay program. It stinks to have someone book a single night and by default snags all 7 days from the short stay availability.