It is absolutely safe to use 3.3 V on VDDD.
Just make sure to turn off the internal VBUS regulator when you are powering VDDD externally.
Thank you for your support.
I have another question.
If 5V is supplied to VBUS_IN_DIS, VDDD will be 3.3V.
Is this still GPIO 3.3V?
If yes, It can reduce the 3.3V regulator to VDDD.
Your understanding is correct, voltage on i/o will be same as vddd=3.3V
However, please note VDDD cannot be used as an output to power electronic devices and should be connected to a 1uF capacitor for the regulator stability only.
You may refer to Page 6, Power Systems Overview of the CCG3PA datasheet for better understanding.https://www.cypress.com/file/363381/download