UCC28C40 voltage on pins: is it OK to have 3.3V on the Rt/Ct pin, when there is zero volts on the Vdd pin?

No it is not a good idea to have an input voltage on any of the pins when the Vdd supply is zero. This will likely cause a parasitic turn on within the part and cause a failure. The UCC28C40 has not been characterized for this but it may be acceptable to add a reverse polarity Schottky diode between Rt/Ct and Vdd and this will protect the chip when Vdd is zero.