I bought one of these Matek boards myself and was having the same concerns.
After poking around with a multi-meter it seems that the Vcc is a direct connection to the battery voltage, so would give ~14.8 - 16.8 Volts if your're using a 4S battery.
Vcc is not a voltage sensor, it will destroy your flight controller if connected directly.
I don't think the board actually has a voltage sensor.
I'd say that the 5V pad you circled in the lower image will be suitable for powering the pixracer, mine gave about 5.03V which is well within tolerance.