On the new Starboards, you are correct, not very many people are using bigger fins than 70-71 ... I am using one 70cm VMG in all conditions from 6-35 knots!
That being said, the Vapor is slightly different. Do you have the new Vapor 2, or the original Vapor? I had the original Vapor for a few seasons (great board) and was using a 75cm cutdown in light/medium winds and a 70cm in stronger winds. I'm not 100% sure about the new Vapor though (maybe ask Ross/Arnon on the Gaastra forum?) as it has a much wider tail. That being said, you are a lighter rider, so maybe a 70cm is fine for nearly all conditions on this board as with an 11m you probably don't need to go such a big fin ...
Let me know about the board and can try to help more :-)
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