8-bit VGA DAC Design Question
The first circuit's output impedance is only ~61&ohm; which is a little low, but the main problem is the maximum output voltage is only ~0.343V when it should be 0.7V.

The second/third circuit is ~77&ohm; and 0.8V (assuming low impedance 3.3V digital inputs) so it should be OK.

The fourth circuit is ~290&ohm; and 0.68V. The impedance mismatch will cause reflections, but they will probably be too small to notice if the VGA cable is short. This scheme has the lowest loading while still producing full analog voltages.

If your digital outputs have significant impedance then you should subtract this value from the DAC resistors to get the correct total. However with only 3 bits the error has to be quite large to be noticeable. 

Precise impedance matching isn't nexessary, but getting the correct voltage levels is. That means the 75&ohm; termination must be taken into account when calculating the required resistor values.