Actually, in general I need the full current range of the Vactrols, 0 to 40mA... I hadn't realized that the digital pots were so limited. I just copied the circuit from the arduino examples page.
Most op amps won't provide 40mA output current, especially quad ones. For 40mA output I would boost the op amp output using a transistor, as in the attached schematic.
If I go the DAC route, then I guess the current to the LED will again be pulsed by the DAC? Or would the OP-AMP flatten then into a constant DC voltage?
DACs produce a steady output, not a pulsed output.
Another possibility is to reduce and smooth the PWM output using 2 resistors and a capacitor, and then use the same op amp circuit to feed the LEDs. But then you would need 15 PWM pins.
I was thinking, could I not use the op-amp with the digital potentiometer voltage divider to keep the current output by the pot wiper very low, while controlling the output current through the LED as the voltage output of the pot divided by R?
You could, but a DAC is a more obvious choice, and you can get octal DACs vs. hex digipots.