The actual dovetail of the Vickers, 3" mortar, Bren etc was the same in order to standardise things but I don't think that the Vickers sight would fit onto a Bren and work correctly with a magazine fitted. I'm not sure that the trajectory charts for the indirect role would equate either although it's all pretty academic anyway
As a bit of an aside, I often wonder what in heavens name were they thinking about.... a 30 round, magazine fed light machine gun in the indirect fire role. We already had heavy stuff to do the job in the 3" mortar and Vickers MMG until the late 60's