We report shape control synthesis of digenite nanosheets (growing perpendicular to the < 111 > direction) and nanowires (growing along the < 111 > direction) through control on the silica vapor pressure in the system. Critical vapor pressures for silica sheath initialization and crystal shape maintenance are determined in two sets of experiments. The geometrically kinetic competition (GKC) mechanism, which was first proposed in our former work (J. Phys. Chem. B 2005, 109, 11585), is also supposed to be responsible for the phenomenon reported here.