As you can see in the
datasheet, pin 4 = VCCIO is the UART (and CBUS) logic level supply on FT232RL (28-lead SSOP package of FT232R). Basically, you need to derive a 1.8V supply (capable of providing at least 50mA) from the USB 5V, and supply that to pin 4. Note: I suspect a simple 16
R:9
R voltage divider (5×9/(16+9)=1.8) will
not work; the current draw will make the VCCIO vary (drop) too much when using a simple voltage divider. And SBCs and FPGAs really don't like exceeding that 1.8V by much, so you cannot compensate by letting the unloaded VCCIO voltage be much higher than the 1.8V. (On my SBCs, there is less than 0.1V of headroom.)
If the end device uses USB power, and generates a 1.8V supply from it, you can connect that to the FT232R pin 4. The datasheet mentions that any 1.8V to 5.25V level works, but that 1.8V to 3.3V "should originate from the same supply as VCC", which would be USB 5V: the 3.3V linear regulator is built-in to the FT232RL, output on pin 17. It can only provide the aforementioned 50mA or so.
If your adapter is one that has a three-pin jumper, where connecting the center pin to one side selects 5V, and center pin to the other side 3.3V, just omit the jumper and provide the 1.8V to that center pin.
Me, I prefer to keep TI TXU0202 (UART), TXU0204 (UART+RTSCTS), and TXU0304 (SPI) in stock. Plus some 74LVC1T45 in SOT-23 for dead-bugging (in air, no board) in a pinch.