Aria only seemed to have $1 VP at bar last time I was there (early March) but the whole system is server based, so it could change at any point from a denomination standpoint.
I've definitely noticed a couple of MGM properties moving to $1 minimum VP at the Bars, however I've still been able to find .25 and .50 at MGM, NYNY,
and Monte Carlo
HET seems to have it across the board.
To the number of drinks, I've usually found it's really just a matter of how long you are playing, mood of the bartender, how well you are tipping, etc. I've had a number of bartenders see if I want a shot along with the drink after being there 20 or 30 minutes or after repeat visits over a couple days.
Best advice I can give with it, is try to hit the same bars regularly, tip when the drinks come, and if you are playing VP focus on playing for at least the first 10 minutes or so. There's a ton of hassle the bartenders get for players that are trying to hit and run, even the grumpy ones are going to prefer to have an actual player at the machine vs someone sinking $5 in trying to score their free beer.