Ok, here we go:
1. At high altitudes green dot speed is near enough your minimum drag speed. So if an engine fails, this is the speed your should fly at. At lower levels, this the minimum speed you should fly for efficiency and controlability in your current configuration. Therefore this is a good straight line holding speed. Adding a few knots means that you are more speed stable in turns. When the message "STALL PROT SPEED" is displayed, the previous is true as long as you add 10 knots.
2. VAC is approach climb and technically this does not exist in Flaps 2. With Flaps 3 it depends on your weight when you made your approach in Flaps 5.
3. Anything between Green Dot (or Green Dot plus 10 with ice) and 230 knots. But if I was flying I'd stick closer to the minimum rather than the maximum.
4. There is no need. I'll make it simple. Fly to your FAF so that you reach Green Dot plus 10 or so one mile before. Select Flaps 1. Slow to Green Dot plus 10. Between 5 and 6 miles, lower the gear and select Flaps 3. Slow to Green Dot plus 10. At approx. 1,300' select your landing flap and final approach speed. Job done.
The important thing in everything above is that what you when do depends on your weight. The lighter you are, the slower the speeds. Fortunately, the aircraft works this out for you.
PM