This is the difference between a $60-80 axle that is made in China, with what level of quality standard? There's a reason the parts store axles are cheap. There is probably slop in one of the CV joints themselves (bearings and races) and that is what is vibrating for you.
Buying one from Subaru or from a reputable shop (like one that rebuilds Subaru axles such as MWE), can be worth it's weight in gold.
On an N/A Subaru, they are easy to change, well at least the old 2.2's are/were. With no turbo/piping in the way, I've changed those axles in less than 30 mins.