the Vmoda is 32 ohms and that's most probably way too low for the macbook. The usual rule of thumb is that the headphone impedance should be 8-10 times or higher than the output impedance of the amp. That would mean the output should have 3-4 ohms or less and apple seems to state less than 24 ohms...
So the sound of the Vmoda is probably a bit colored when directly connected to the macbook and will sound different when when the macbook just drives the O2 with >10k ohms input impedance.
The O2's output impedance is about 0.5 Ω according to the blog.