but the big split between recommended ‘unstick’ or Vlof (more appropriate terms?) and blue line meant there was a short period when you could be airborne but seriously compromised if an engine failed.
That short period after airborne and attaining blue line speed (usually about 10 seconds) applies to many light piston and turbo prop twins.
The RAAF had that problem with the introduction of twin jet aircraft like the Canberra bomber and Meteor fighter. Depending on weight typically the Canberra had a recommended lift off speed of 110 knots and a safe asymmetric speed of 160 knots at full load. The Meteor was similar. These numbers are off the top of my head and from memory so could be inaccurate.
Either way there could be a 40-50 knot spread between recommended lift-off speed and where, if an engine failed, the pilot had to make a decision to either reduce power on the live engine to help maintain rudder effectiveness (directional yaw) and hope the aircraft could still climb out albeit reduced rate of climb - or running out of rudder and risk rolling on its back. That spread of speeds was known as the dead man's gap.
The only other solution if an engine failed below single engine ‘safety’ speed was to force land straight ahead
under control and pray there were no obstacles.
In the case of GA light twins, the spread (or dead man's gap) between POH lift-off speed and Blue Line is usually around 10-15 knots. You either accepted the miniscule risk of losing an engine inside those few seconds between airborne and single safety speed or you pulled up stumps and went home. .
So much depends on the flying skill of individual pilots and their technical knowledge of the single engine performance of each particular aircraft they flew.
Some pilots had their own personal ideas such as holding the aircraft on the runway until lifting off at single engine safety speed or ‘Blue Line’ speed. That could bring in other factors such as a high speed abort if an engine packs up just as you reach blue line while still on the runway. Examples are does the aircraft have anti-skid brakes? Is the runway wet therefore less braking efficiency?
Faced with so many variables some pilots may prefer to stick with POH recommended VR or lift off speed and accept that, unlike airliners certified to have a V1 and Vr capability, there will always be a window of indecision of a few seconds until reaching airborne blue line speed.