Can I use a DAC to tune a VCO?
Can I replace R1 with the DAC output from an AVR microcontroller?


Yes you can but be aware of noise on the output of the DAC especially when changing DAC values - you get a bit of capacitive coupling from the clock and this might require a bit more filtering than just the 270 ohms and 2200 pF will bring about.

For this reason, it's not generally recommended to directly couple a DAC output directly to the TUNE input.

ADDED SECTION
Given that the "so-called" DAC output may be PWM output with possibly some filtering, I would advise to make any filtering as decent as possible. The PWM frequency needs to be kept as high as possible and you will probably need at least a 2nd order low-pass filter with a bandwidth of sub 10 Hz (or better).