Just don't foget that Maura could've parked (no one has said otherwise) right in front of the dorm she was working at (Melville) as well. There is a parking lot (That I believe is) reserved for security/staff.
If she did park there, it would've taken her 10 seconds to walk to her car from her post.
I've asked around a little bit, but haven't really sought out if she would have been able to park directly in front of the dorm, but in any case, I still think that this is unlikely, because she almost certainly would have had to put her car back in Lot 12 by morning. If she were going to permanently park by the dorm, she would have a sticker for that lot (or, perhaps, a placard to hang by the rear view mirror, but I've never seen mention of this being found in the car). Furthermore, depending which way she would have left Southwest by car, she would have to go through 2 or 3 traffic lights, potentially adding more minutes onto my rough estimate. I only spent about 10 seconds at a red light when I did this. I also measured time from driving by the dorms, even if it takes 10 seconds to walk to a car by that building, figure another 3-4 for getting into the car and starting it, half a second to decide not to let it warm up for a bit on a winter night, another, say, 5 seconds to put the car in gear and pull out onto the street. Taking all of this into account, my "minimum" travel time would hit the 6-10 minute range pretty easily. This also assumes that she hit Vasi while headed away from UMass. The sidewalk (at least today) is on the opposite side of the road from her direction of travel, as is Mattoon St itself. (I don't recall if Vasi was found on Mattoon St. or in the middle of Triangle St. at Mattoon.)
Also, for me, it comes back to there just being no need to go in that direction. A list of food/drink choices that are closer/similar distance, but may have required her to time her break a little differently: Vending machines, various campus dining options in Southwest, other campus dining options (some are open after midnight today, no idea about 2004), Amherst center (today, there is at least one pizza place and one calzone place open late, not to mention bars), other Dunkin Donuts shops and gas stations on University Drive, and a little further out is a main drag of sorts in Hadley with a small mall, Target, Walmart, diners, fast food places, etc.