Can FPGA VGA be used as DAC?
For a better quality of producing RGB signals you could use 4,5 or 6 digital signal (FPGA pins) and use a resistor-network (as a DAC). Take a look at this schematic which is used in an FPGA evaluation board (ALINX-AX309) and check out how it produces the VGA signals:
https://github.com/alinxalinx/AX309/tree/master/SCH
And you can also produce an analog signal with PWM. It can be implemented by a counter and a comparator in FPGA. Then filter the output PWM signal simply by a RC filter to obtain a smooth analog signal.