There is no power-on sequence requirement for USB-Serial devices (CY7C65211/213/215 and its corresponding A parts).
So VCCIO/VDDD can be powered before VBUS.
The VCCIO/VDDD will power the core logic and IOs, where as the VBUS will only power the USB PHY. Absence of VBUS will disable the pull-up on the DP line (no current will be drawn from the VBUS line).
You can keep the VCCIO/VDDD powered all the time, provided that the current consumed by the core and IOs will not be a concern.
Else, you can use a 3.3 V regulator on the board and use the VBUS line as input to the regulator and provide power to the VDDD/VCCIO when the VBUS is powered.
Refer to the schematic design in CYUSBS234 DVK.
Hey Yatheesh, thanks for the answer!
What current consumption may I expect for the CY7C65211 with VCCIO active at 1.8 V and VBUS removed?
The datasheet states 5 uA in USB Suspend (which would not be a concern), does that also apply for VBUS removed?
Would I have to pull nXRES low to achieve that?
Yes, when the VBUS is not present and the suspend feature is enabled using the USB-Serial Configuration utility, the device will go to suspend mode and draws a typical of 5uA current. You can check if the device is in suspend mode by monitoring the SUSPEND pin of the USB-Serial Device.