The way most keyboard players add vibrato is to turn up the mod wheel, and inject some LFO to change the oscillator pitch periodically. That's fine, but consider guitar players—they add vibrato by moving their fingers on strings, which gives a more human quality than using an LFO.
So, try your hand (get it?) at doing vibrato with your fingers instead of using the LFO. This also frees up the mod wheel to do other, perhaps more interesting changes (see the tip from May 9, "Get Imaginative with the Mod Wheel."