I guess you were reading the datasheet of the PL2303 not the PL2303HX because in the datasheet of the PL2303HX it is stated that pin 4 VDD325:
RS232 VDD. The power pins for the serial port signals . When the serial port is 3.3V, this should be 3.3V. When the serial port is 2.5V, this should be 2.5V
You can check it out for comparison and there's also a schematic at the end, I just want to know if it'll work with 5v directly.