I understand the Vivid inks will trash the print heads on the older, non-coated, machines like the 7800 and 3800.
I did load the K3 ink set in my K2 machine, an Epson 7600. No problems, but no LLK channel either.
Not 100% sure I believe it, but not worth trying for the small gain in gamut.
Yes, to get the x880 ink set you will need to wait for a 3880. Or wherever they are going to go, they have a newer ink set than the Vivid with the 7900/9900. Doubtful they will bring that down to the smaller machines though.
Best,
Michael