A relatively coarse grind works fairly well for me, but something finer than a french press grind. I gather that this is a "filter" grind. You can experiment with dose, but something like 10g per "cup" (ie. 120mL in Hario speak) is a good place to start.
The key thing is to make sure that you start off with boiling water in the bottom. Heat it so that it bubbles without the top part locked in. You should, however, leave the top part in the bottom, so that the chain hits the base and provides a nucleation site for bubbles to form on when boiling the water. Once it comes to the boil, lock the top part into the bottom and watch it shoot up north. Make sure that there is still a little bit of water in the bottom, above the flame - this prevents the glass from cracking. If there are large bubbles and a lot of turbulence in the top section, you will need to lower the flame slightly. Throw the coffee in, stir, wait, cut the flame, stir and enjoy the show! I find that cutting the flame after about 30 to 45 seconds is about right.
Im sure that theres an art to vac pots that mirrors all that we know about espresso.
Hope that helps,