I'm not very good at audio on the technical side but what I do to add vibrato is to click the instrument in the track (the big block with the name of the instrument) so you get the channel settings window for it.
Go to the INS tab (instrument properties)
Click on the RES sub-tab
Where the wave is shown, below are a few knobs and you just move the AMT (amount) knob all the way up, and then play with the SPD (speed) to get your vibrato effect.
Clicking the TB and Global switches set your vibrato on time with the music which I think is more if you're doing a Dubstep "wub" sound.
Hope that helps.
