You have to choose the vacuum thickness such that the physical properties of your interest do not virtually depend on the actual value adopted. So, you have to do a couple of calculations for different thicknesses to figure out what is the thickness that suits your requirement for the accuracy of the calculation.
Regarding the dispersion direction, I guess this is your choice of making a nanoribbon in the direction of your research interest as well as setting all the other nanoribbon parameters.