I'd drive an opto-isolator's LED directly from the logic output of ST3232CTR (VCC=5V). I'd set current with a serial resistor (Vcc - R - LED - 3232's output. Output=L to light the LED up).
I checked "DocID7454 Rev 12" and found fig.5 and 6 with Uo=f(IoL) graph for VCC=5.5 and 3V.
As current is widely changes by the Vcc, I'm looking for a figure with multiple graphs for different supply voltages.
Can anyone help?