This appears to be new behaviour but I could be mistaken, recently I have noticed that my vt60 changes from "Custom" which I have configured to "Home Theatre" whenever I watch a Blu-Ray. I prefer to keep everything on the same video mode, at least until I get the TV professionally calibrated. I have all my devices plugged into my Denon 1912, which then connects to HDMI2 on my VT60. My blu-ray is a Panasonic BDT210, I have tried turning off the Viera Link, but that made no difference. For the life of me I can't figure out why this is happening, I never noticed this behaviour before, but can't figure out when I could possibly have changed that could have introduced it. Also, while I'm at it, on my TV in the Viera Link settings, the speaker output changes back from Home Theatre to TV at what appears to be random times.
All three of the devices I mentioned above are at the most recent firmware levels.