This is a lot harder question than you think, Adi.

Is the vocal already compressed, or is it 'natural'?

Is the tune backing (and the vocal) very dynamic, or does it 'sit' at one level (roughly), most of the song?

I guess the easy answer (although it isn't easy to achieve!) is 'as long as the vocal never obscures the backing, and the backing never obscures the vocal, you've got it right'...

Once you have mixed the track, let your ears 'relax' for a few minutes, take a short break, make tea, whatever. Then re-listen to the track VERY QUIETLY. Can you still hear the vocal all the way through? Can you still hear the backing? Now listen to it at slightly higher than normal level. Do you hear the same balance?

If so, you are probably on the right track. Now get out a CD of similar kind of music. Listen to the vocal balance on the pro CD. Does it sound similar to your track? If so, you've got it.... If not, back to work!

Vocal compression is probably the key. Try a mild amount of gentle vocal compression (most software compressors have a few presets for vocals) and see if that helps 'sit' the vocal in the track better.

But there is no 'magic bullet'. Each vocal, each track, all have to be treated slightly differently. Just use a pro CD as a listening reference (in a similar style to what you are trying), and you will always have a good reference.....