Haven't tried the newer Z fins but I know Ross and Arnon were using ZE 70's in the original Vapor in most wind conditions. Also Ross was using a Kashy 70 XS (as well as Steve Allen back when he was on Gaastra).
I think you'll find a 70 of anything relatively new - ie, Z-Fin, VMG Blades, Virus, Boss, Kashy etc etc will do quite well for nearly all conditions ... if you want a touch more power in really light winds (<10 knots) a 72cm cutdown would help. I don't think at your weight you will need anything bigger ... the original Vapor is quite a free sailing board, it rails very easily and gets a lot of lift from the fin with the outline, so you don't need to put massive cutdown fins in it! Make sure you keep the mast-track around the middle in this board - i see lot's of guys using it further back than middle, but I think that stalls the board as it already lifts the nose enough just from the aggressive rocker it has!
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