I'm sure those with much more knowledge than me will have the proper
answer, but when I had an LX TV + beolink 1000 I had a cable link to my
B&O stereo system and the AV button was used to transfer the sound
from the TV to this system and thereby listen to TV (mostly music
broadcasts) via the stereo system.
If you want to use a DVD player then I would suggest taking the scart lead from the player and plugging this into the TV and press Video tape on the remote. The TV will just treat it as another signal coming in. Can't remember now whether the TV had to have its scart socket programmed in some way to activate it but a look through the user guide(s) should tell you.
Now if you want to use both a VCR & a DVD player that may present a problem.