I also had this issue, with up to 28% of cpu being sucked up by VTDecoderXPCService. I shut down possible problem applications one by one: Chrome, Thunderbird, iTunes, etc. with no change. Then I remembered that Full Deck Solitaire had a video background and changed it to wallpaper, and voila, VTDecoderXPCService shut down and did not come back. It also fixed some of the issues I was having with speed, so I think the hit of ~30% of the cpu was making speed issues apparent. (spinning balls, spinning circles, etc.) Even filling in the Open With program list was sometimes taking 8-10 seconds or more when clicking on a TXT file! I have not checked if this helped that issue yet.
Based on this, you might check anything that may have a video loaded/running, even in the background. With the ubiquitous videos on all web pages anymore, this might be an issue where I go back to the DOS days and Quit the browser unless I am going to use it. Caveat: I have not checked in depth to see if having the browser open brings back VTDecoderXPCService, but a quick check does not seem to be a problem.
This is on a mid 2010 MBP with 2.66 GHz i7 dual core and 8 GB ram running 10.10.5 Yosemite.
Hope it helps.