First of all: This falafel stall is not there every day. If you're unlucky, you can get very good falafel at King of Falafel, just around the corner in Urbanstr. 67, or at the sudanese restaurant NIL on Weserstr./Reuterstr.
Next thing: The sauce is vegan. When I asked if it contains yoghurt, he answered: "No, it's sesame sauce. It's vegan" (I guess it's not the first time someone asked)
They serve decent falafel for a decent price, but then there's dozens of snack bars in Berlin doing the same, so it's not worth it to drive all the way to there just for a falafel.