Anytime you have an intermittent fault indication like that, you should check the battery, it's main cable connections and the charging voltage first before doing anything else.
VSS is "vehicle stability system", commonly called "the nanny".
It has several components and if that continues you probably need a dealer to connect their diagnostic computer to try and catch which part is going bad.
AND.....if it is still under factory warranty, you should NOT mess with it yourself, except maybe checking the battery stuff.