20752 and 11144 are both NEC1 device 0, so either should work. I'm not sure why 20752 didn't work for you.

Regarding 31517, as you say, it isn't a valid setup code for the Comcast remote. However, in spite of what the Vizio documentation says, we don 't expect 31517 (NEC1 device 131.95) to work with either the 510 or 215 sound bars, and that's what the first post in this thread points out. It does work with the VBS200 soundbar.