If anyone else ends up here like I did.
To drive the DIM Control pin without pulling from Vdd, try this.
This will keep the micro isolated. This assumes that you use a dual optoisolator, and that the CTR is the same. To ensure they ARE equally matched, you can buy the more expensive IL300 or LOC110 - built especially for this purpose.
Also do note: if you are going to make a DUAL dimmer, then do NOT connect the 'GND' between dimmer A and dimmer B. They must be separate.