A previous home had vinyl framed Anderson windows. After cleaning the sides, they were "tuned up" with dry teflon spray. It sure made the windows slide nicely. Was told to avoid silicone on the vinyl. No idea what all the other solvents in your "E408" might do to a vinyl window frame, so maybe test in a non-critical spot to avoid messing up a something expensive.
TomKro
TomKro