You should always leave "hole" in the
mix for vocals. You can do that with volume, EQ, effects, compression or placement. Certain instruments share the same range of the voice, especially guitar. Consequently, the vocal ends up fighting for space with the guitar (or in your case, the synth). That's why guitars are often mixed to the left or right, as the vocal is usually mixed dead center. Try spreading out your synth tracks to the right and left, and you will hear the vocal track become more prominent, without having to add any EQ at all.
How much EQ you use on a vocal depends on what you are trying to achieve. Most engineers use "subtractive" EQ, i.e. - rather than boost the high freqs, cut the low freqs. You get better clarity and you don't overload the track. Vocals have a lot of "mud" around 250-500 Hz. Cutting there with a graphic or parametric will clean up a vocal track nicely.
Here is a chart that shows the frequency ranges of various instruments, and the human voice. You can see which instruments occupy the same range as the vocal.
http://www.har-bal.com/index.php?/frequency-chart.php