You can't really do this in Vegas and exactly hit the values you want. The values seem highly unrealistic anyway. To have an overall average of -8db with peaks at -6db would mean an incredible amout of compression, which would be pretty much unlistenable. Could you explain a little more about what you're trying to accomplish and why you have picked these values?
Adobe Soundbooth seems to work for this, there is an FX "everything same loudness". I just tried it out and it really did seem to make the entire wav file within just a few db. That would give you a good start-point, then adjust the amplitude to around the -8 area.
Well.... you SHOULD be able to do this in Vegas with the compressor. It won't be the easiest thing and you may have to run the audio through the compressor a few times. As John said, you won't get the precise numbers you're looking for, but you should come close. The -6 is easily attainable... the -12 you'll have to diddle a little... but the -8.... that'll be a ballpark reach
The ear can tell 3dB or more difference. Old time telephone recordings,
"the number you have dialed....." had 6dB variance between phrases.
So specs for this is likely for automated response tracks.
Boring to listen to but needed. PaulJG am I close?
Simply throwing compression at this is not the way to fix this.
Compression is not some automatic gain control at all.
If the problem is that the speaker is raising and lowering their voice or move away from the microphone then you need to fix that problem first. Do this using a volume envelope on the track.
Then you'll need to reduce the dynamic range of the speech using a compressor. You need to put this onto a buss, not in the track header as in the track header it is before the track's volume envelope.
If you get this setup properly in Vegas you can watch what the compressor is doing by leaving it open in a window. It's very informative. You can also adjust the level out of the compressor using it's output slider. Getting all of this done correctly is quite important when you are applying heavy compression like these specs are calling for.