I think there is definitely a distinction from karaoke. The voice is an instrument just like the bass and singers/vocalists train and practice to become better just like every other musician would. It sometimes gets forgotten though because everyone has this instrument and, like lowdowner said, it's not a physical object. But there has to be a line between people that dedicate years to perfecting their singing voice and people that sing on karaoke every Friday night. I think its a case of, if you are good at karaoke then you have the potential of being a vocalist (in a similar sense of having good hand-eye coordination and rhythm, would be a good start for a drummer). But just like all instruments there's so many different techniques to learn and ways to improve stamina and tone etc which would require years of practice and hard work.
Having said this, I am not a singer and I have nothing against karaoke! I'm just conscience of some people thinking their craft isn't fully appreciated