Hi RichPa,
There is a good write up about a B707 accident G-APFK, which crashed due to being below VMCA with level wings.
http://www.aaib.gov.uk/cms_resources...8%20G-APFK.pdf
http://www.aaib.gov.uk/cms_resources...K%20Append.pdf
The 707's VMCA increases by 40 kts (from 5 degs bank into live engines) when wings are level .
If you run out of rudder authority the aircraft yaws. On a swept wing aircraft the outside wing develops more lift, and you can run out of aileron authority.
The VMCA is demonstrated with 5 degs bank into the live engines with full rudder and constant heading. The aircraft will show the slip ball out towards the lower wing. Because the C of G is forward of the C of Lift there will be an additional yawing component helping the rudder.
When the wings are level, that additional yawing couple is reduced to zero.