As far as installing vlc, or most anything, I always do the custom install, just to see what there is in options.
With vlc there is an option to uncheck making it the default player.
As far the KMPlayer. I like it a lot, but I opt for just hitting a song or opening a folder and click play without all the extras. But the extras in Km are nice to play around with. And it will play anything as well I think.
WMC, VLC, and KM play audio and video files fine....so far.
For me, it's the ease of playing songs and sometimes playing around with options, but mostly I just want music.
I'm glad they can all play on the same computer, bypassing quicktime and realplayer and wmp which we were stuck with years ago.
We can even play more then 1 song at once just for fun I reckon.