The buses from Mendoza to Vina/Valparaiso (many) take the same time as to Santiago, they just take different routes at the turnoff just before the Chilean town of Los Andes. Take the day bus rather than the night bus to see the scenery, especailly in the high Andes.
A couple of hours in the mountains? Well, you could spend an hour at the interpretive walk viewpoint to Aconcagua (the highest mountain in the world outside of Asia). The Park entrance is, unfortunately, about a 10 minute walk up the hill after the joint Argentina/Chile customs and immigration spot where you have to check in. I would
just get walk out of the Immigrations after checking out of Chile and into Argentina, and walk the 10 minutes up the hill to the park entrance. Pay your roughly 25 peso entry fee for the lookout, and take the hour long walk up the road. If it is a clear day, you have a great view of the top of Aconcagua. Also, they have a beautiful little pond on the trail you could picnic beside. Any food would have to be taking in surreptiously because technically it is illegal to take food in because the Argentines leave piles of garbage behind when they picnic. The last milkrun bus to Mendoza leaves the park gates at 4:45 pm (other side of the highway, flag it down, gets into Mendoza about 8:30 pm, it only costs about 25 pesos.).
The other option is to further walk the half-hour down the highway to Punte del Inca, a historic and natural site (a natural Sulphur bridge over the river and inaccessible hotsprings, that is worth seeing for about 10 minutes (other than that it is a hideous collection of souvenir stands and sheety "restaurants") The last milkrun buses to Mendoza from Puente leave at 4:45 and 8:00 pm.
In terms of your baggage, you could either work out a deal with the independent baggage handlers in the customs to watch your stuff, or take it to the park gate office, as the wardens there are good about keeping an eye on your bags.
Uspallata is not that bad to spend an evening in, the restaurants are fairly decent, but not great, but boring as hell. There is the Hostel Uspallata lodge that is a miniresort that would not be bad to spend a day with if you want to arrange horseriding or just hanging out at the pool.
Forget Potreillos, just look at the lake as you pass by in the bus.
The real nice hiking is in the Cordon de Plata range south of Potreillos, but the trails are complicated to get to and you need to arrange a trip there with one of the Mendoza tour agencies on Las Heras street.
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Oh yeah, agree with those that say Valparaiso is unforgettable, an historic port city with a charming old town worth spending a day walking around. Get a bed-and-breakfast on Providencia hill above the old town, absolutely charming. Vina is just a slice of California. Instead, take the local buses up to Cocón along the ver picturesque coast highway for about 15 kilometers. The beach at Cocón has a lot calmer water, the beaches in most of Chile have very cold water and dangerous currents.
If you have the money, rent a car and drive along the coast north of Vina,
Get the prices at the restaurants and bars before ordering, many like to overcharge gringos after the fact in the Vina area.
