What SiliconWizard said.
The VDD_TARGET is not only an input, but on the F4 Disco board it's not used at all: the only connection are a 10K resistor to GND and a diode to...nothing (unpopulated 100
resistor).
The voltage reported by OpenOCD is not related to that pin (TBH, I don't know where it comes from...
).
You can get 3V (not 3.3V, but they are OK for the BluePill) from jumper JP1 on the Disco (or any pin marked VDD or 3V if JP1 is not removed).
Another important point: the ST-Link onf the F4 Disco is not V2.1 but V2, the correct interface driver must be used (interface/stlink-v2.cfg)
See the attached pictures for a bad photo of a working setup, and related OpenOCD messages.
It's on Windows, but there's no difference.