The bus trip is great! I enjoyed it so much. It was seven hours from Vina. For me that's just right, not to long not too short.
(I've done the Vina to Arica 30 hour bus ride a few times, now that is torture:-)
The mountains are amazing. Get a window seat and you get a great view going up to the pass, it's about 12,500 feet at the top. Then again, if you're scared of heights maybe sit in an aisle seat

it can be a white knuckle experience. The road is a long series of switchbacks and there are no guardrails
It was about 30% cheaper for me to go to Mendoza stay in a hotel over night and come back than to renew my visa in Stgo. I've done both.
I don't know what you require hotel-wise but there was a hotel on a corner across the street and to the right from the front of the bus station that was OK. 9k peso's overnight. It was OK for me for one night, it was nothing special, cheap, clean.
Enjoy the trip, take plenty of pics!