The meaning and role of the phase in antenna measurements with VNA
VNA measurement results are just complex numbers. Complex numbers can be expressed in either "magnitude (Z) + phase (P)" or "resistance (Rs) + reactance (Xs)".  

You don't want to mix these complementary notations of complex numbers. So Z=50 and Xs=0 is not what you are looking for. 

For SWR=1 with a load of Rl=50 Ohms you need Z=50,P=0 or Rs=50,Xs=0, which is the same.