yea - not a trivial thing to edit - and it isnt soft ware that you need but a tonne of patience and a clear head.

As with most things there are lots of ways fo doing this with varying results and difficulty level.

First thing I would say is that if you are using the value version of vegas with only 4 tracks available it may be a struggle.

You need to break the thing down into its parts. You want to see a real tv, then one track will feature a pic of a tv. The track above should have any video you want on the screen resized with the track motion tool ( one way) so it fits in the screen area.

Above that the volume thingy. I suggest you film that for real off a tv as you adjust the volume - I always look for the easiest way and often filming something 'real' is easiest.

The volume changes are easy - just use a volume envalope.

As I said, not a cinch for a noob but struggling through this stuff is what makes you a better editor, technically at least.