Cabin top mounted travelers rarely work as well as end boom sheeting in terms of vanging the boom. Generally it is is difficult to get the downward component at the leech out of them that you can with end boom, or nearly end boom sheeting. Vangs become critical in controling leech tention and twist when reeching and preventing the boom from bringing down the rig by hitting the backstay when jibing in heavy air.
I may be missing something but I don't see how a cunningham and outhaul is a substitute for a vang since neither impacts leech tension.
Boomkickers and rigid vangs are a nicety that adds little for most cruisers,
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Curmudgeon at Large- and rhinestone in the rough, sailing my Farr 11.6 on the Chesapeake Bay and part-time purveyor of marine supplies