Donal,

A very good description of a Vangs usage.
When you ease the main sheet past the angle of the traveler, the vang
maintains boom height and sail shape.

That is why I asked about vang tension. Just enough tension on the boom
to start to ease the main sheet when it is close hauled. Any more
tension will force the top of the sail to hook

By the way Donal is you don't have a vang you can set a barbar haul and
get the same results. Set it on the sheet and to the toe rail and you
can still trim the main

Ole Thom