A circuit to prevent parasitic powering of FT232RL by MCU, will it work?
A usual way to solve it is a voltage level shifter. People seem to offer mostly bidirectional ones, but I like using uni-directional shifters, as they are more resistant to noise.
For example, SN74LVC1T45 is 1-bit dual-supply unidirectional level shifter. Its datasheet specifically mentions what you need:

"V_CC Isolation Feature – If Either V_CC Input Is at GND, Both Ports Are in the High-Impedance State"

http://www.ti.com/product/sn74lvc1t45