The interesting stuff starts around page 45.
TL;DR: An USB-C charger is not allowed to exceed ‘default USB power’ if the device at the other end of the cable is not able to negiotate the need for higher current (see page 50 above). Vim2 seems not to implement this (so not getting more than 500mA or 900mA is the only reasonable choice with a real USB-C PSU) and the PSU sold on Gearbest seems just to be a dumb one simply providing up to 2A.