The Chosen widget does not appear on select elements inside vertical tabs on node edit forms. The Chosen container (i.e. the .chzn-container div) has a CSS width property of 0 set on it. Unsetting this CSS property makes the Chosen container appear as expected.
Tested with Chose module version 7.x-1.0, Chosen library version 0.9.9, and Fieldgroup module version 7.x-1.1 ( to provide vertical tabs in node edit forms)