[erickejah (OP)]

Why I cannot use a impedance vector diagram to calculate the  phase angle in a of a parallel RC circuit?
[diagram=383_0]

[lyner]

If
V = ZI      (which it does),
where Z is the impedance,
then you definitely can tell the relative phase between current and voltage in a circuit. I assume that's what you mean.

The phase will change from +90 to -90 degrees as you vary the frequency and passes through zero at resonance - when the shunt X will be infinite, for a parallel circuit.
In a series circuit, the series X will be zero at resonance.