Serato VSL doesn’t support syphon yet, so you’re going to have to use Mix Emergency instead (http://www.inklen.com/mixemergency/).
I had to buy ME after i realized Serato VSL doesn’t support VSL, and support in general was lack luster, pain.
So Serato > Mix Emergency > Syphon > Resolume. Here’s a tutorial using Modul8 (http://themidithief.com/2011/06/syphon-input-in-modul8/) the most important part is that you grab the qcRehab files.
Until Serato VSL builds syphon into their software, Mix Emergency is where it’s at.