[Slakah]

You need to pass "self" as the first argument of FauxScrollFrame_OnVerticalScroll.

So in your XML (never used XML so this is a guess) you replace:

<OnVerticalScroll>
   FauxScrollFrame_OnVerticalScroll(16, FFSlashScrollBar_Update);
</OnVerticalScroll>

with

<OnVerticalScroll>
   FauxScrollFrame_OnVerticalScroll(self, 16, FFSlashScrollBar_Update);
</OnVerticalScroll>