I must confess I am in love with my vacpots. I have the Santos 10 cup and also a modified Hario 3 cup for those just me times.
As for your grind size it is relative to you brew time. I go for a medium grind, coarser than a filter machine but finer than plunger. I use hot water to start and heat with a medium heat. Once half the water has risen into the top I stirr the coffee and time 2 1/2 mins. I take it off the heat and it takes 1 min to syphon back down. Certain coffees Ill grind finer or coarser but that is trial and error.