If you haven't had this problem, then you are either an instinctively brilliant mix engineer or you just haven't noticed it yet.
Listen to one of your completed mixes. Does the vocal sit nicely into the backing track – not too loud so that it sounds disassociated with the track, kind of floating above it, and not too quiet so that on occasion it can't be heard clearly enough?
Listen to some CDs that you have bought. Notice how on some tracks the vocal can be loud, on others it can be quite quiet, but always it seems to be at the right level. This is due to the skills of professional mix engineers who know how to make a vocal sit in the track, and can choose to make the vocal dominant or the track dominant, but at all times the vocal is clearly audible and never too loud.
One method of doing this is to carve out a 'niche' for the vocal. The human voice tends to be strong in the frequency band around 3 kHz or so. Solo a vocal track and apply some mid-range EQ boost to it. Now sweep the frequency control to see where that boost has most effect. Almost certainly the vocal will benefit from some boost around 3 kHz. But also there may be a secondary resonance lower down that could use some emphasis from an additional band of mid-range EQ.
Practice with this, preferably with different vocalists. Get a feeling for how much EQ is enough and how much is too much. Notice how you can increase subjective level with EQ without touching the fader.
Now let's turn to the backing track. You need to mix the backing track to a stereo bus, and apply EQ to the bus so as to equalize the backing track as a whole.
Look carefully at the vocal EQ you have been experimenting with and set the same frequencies for the backing track's EQ. Set the gain controls for the EQ to exactly the opposite of the settings for the vocal. So where the vocal is in the 'plus' zone, the backing track should be in the 'minus' zone.
Now mix the two together.
You will find that like magic the vocal can now be lowered or raised in level, and over a wide range of levels it will sit very nicely in the track. You can have it quiet but audible, or loud but still part of the track as a whole.
Of course, there is an artistry to this and many more subtleties than can be described in mere words. But starting with this simple technique you will soon find that setting the level of the vocal is no longer a problem.