If you have a horizontal vessel, you can change the angular position from 0 to 180 degrees. 0 degrees at the top of the vessel, and 180 at the bottom. If you have a vertical vessel, you let the longitudinal position be 0 (bottom) and the BDV, or PSV if you like, will be placed at the bottom.
The water, HC liquid and gas volume fractions could indeed be obtained from these initial parameters. However: Small deviations in composition would imply large deviations in volume fractions, and therefore also in total mass for the segment. The inlet composition is not necessarily the same as the global segment composition. Liquid and water levels are widely measured and serve as more consistent parameters for calculating phase volumes than the composition. In conclusion, VessFire finds the phase compositions from flash-calculations, but not the size of each phase.
Check your file path. If you have the period symbol in your path, remove it and try to run the case again.