According to Mason's "British Bombers " pg 324 Chadwick always had doubts about the Vulture and he began to draw up designs around the Sabre, the Centaurus and the 4 engined Merlin version aboutthe time of the manchester's first flight.
Can't find out if it was ever tested tho'
Wikipedia states" Manchester IIA - As Manchester II but with 2 x Bristol Centaurus. None built."
The Centaurus first ran in 1938 but didn't go into volume production until 1942 so it could have been available for a test version of the Manchester