EDIT: It looks like Salvage will be a PvP event every other week, as it is now back.
After not knowing what would count as a capture, we figured out that it is the deploy stat at the end of the game. The achievement should pop after you have done 20 deployments. The achievement will pop mid-match.
A deployment counts for everyone who is in the circle when the progress bar completes. If you feel like you have done 20 and the achievement hasn't popped, like other achievements in this game, you will need to go to the settings and switch profile, followed by selecting your profile and character.
Since The Dark Below DLC released, it has been a bit unclear if this achievement had been locked out to anyone who didn't purchase the DLC. After a few confirmations in the comment section, this should still be obtainable.
QuickMythril kindly checked it out:
i am looking at salvage right now on the crucible map. there is a regular one on the bottom and a "dark below" one in the top right.