http://www.adsrsounds.com/category/serum-tutorials/
In this tutorial, Echo Sound Works shows you two methods to create a vibrato styled effect in Serum.
Some soft synths like Massive come with a dedicated vibrato control. Others like Serum or even Sylenth do not. Luckily with the drag and drop Modulators in Serum, creating vibrato is really easy to do.
I personally like modulating the depth of the Coarse Pitch with an LFO. I like using the preset "A little Movement' for the LFO curve. If you use this method make sure you modulate only at a positive depth on the coarse pitch!
Cheers
Echo Sound Works