As of 1978 Boeing and the industry started to include VEF in their performance info' and references in the V speeds.
VEF is when the engine is failed prior to V1 on the tests(constructor selects this speed).(sometimes as high as 3 sec's,to SEE fail'on TRIJETS)
V1 is the reaction Action limit(stop or go)after the fire/failure HAS been recognised after VEF...The first action.or no action,must be DONE by V! to stop in the Runway distance..
The Airbus FCOM stipulates the perf complies with FAA25..
FAA 25-107 stipulates that the pilot must have RECOGNIZED the failure and ACTED by V!
Some pilots 'feel' the 2 second time stipulated in 25-109 means that they can DECiDE at V!...The 2 second limit is to the DISTANCE for the ACC/STOP distance.....Decisions are BELOW V!.
Cheers..
VEF is genarally about 1 second(5knots)below V1.....
