I've just upgraded to QGIS 2.12.3 - and hoped that the issue which was really time-consuming in 2.12. <3 could have been solved ...
On duplicating layer - with tons of joined tables and virtual fields based on complex expressions - any of these virtually calculated fields are lost. This issue is already listed on https://hub.qgis.org/issues/14191 - unfortunately just as bug with normal priority.
I think this feature is essential. are there any workarounds ?
(i know - "Save As" would be a last option. but once more i do not want to duplicate data - which is one of the benefits of join-concept.)
... i have just tried to do "Save Layer as Definition File..." which seems to work fine concerning the virtual fields but (!) did "forget" the label expression i've used and one single table join ..