isn't Vmca based on the rated thrust
Indeed .. but, in the event that the operating throttle is slammed open due stress or whatever some engines can put the pilot into an awkward situation if the TO has been predicated on speeds down near whatever the Vmca happens to be on the day .. looking at the accident I referred to earlier .. the engines had been modded to the maximum available (ie they put out lots of thrust), the pilots were thought to have had a confusing problem with one engine on TO which was associated with a significant thrust loss, almost certainly they firewalled the remaining engine .... and ended up in a fiery heap in a ditch off one side of the runway head. It is not just a matter of simplistic one-liners in emergencies .. he who deviates from the published procedures takes on a substantial, even if only potential, level of risk which might be associated with the deviation.