compression and reverb help vocals sit better. I compress the vocals some so I can bring them down in the mix and still be heard the whole time. Reverb helps them sound like they're in a real space like the rest of the instruments.
Another thing that might be happening is the vocals are competing with other instruments in the same frequency range. Because of that you're having to turn them up more and they are standing out. EQ a space for them by cutting in the primary vocal frequencies on some of the instruments (guitars get in the way a lot), and a slight boost in those same frequencies on the vocals. It sounds like it'll make them stand out more...and basically it will...but it will let you mix the vocals in at a lower volume so in the end they won't stand out more.
As far as backing vocals, I keep them really low, really compressed and really reverby that way you can keep the volume low, but they can still be heard some. If you want them less far back you can use less compression/reverb. If you want backing vocals like Def Leppard, I can't really help.
