Since there is no such thing as "Vmca" for a single-engine aircraft
Not often considered but a large engine single aeroplane can have a situation similar to Vmca at low speed and high thrust. I understand that some OTE exercises in the military have considered this.
You won't stall/spin as soon as you slow below VMCA in a MEL
.. but you may be quite close to stall at book Vmca. Also, keep in mind ASI errors and the reduction in real world Vmca for most lighties as altitude increases.
Be careful .. for there be dragons .. best to stay right away from Vmca as there is little value in playing about with it.
even pilots apply full rudder they would still cant keep the aircraft on the centerline.
The aeroplane won't stay on centreline with a failure at low speed. Vmcg is defined in terms of a maximum deviation from centreline. As the failure speed increases, the deviation can be constrained to a smaller value.
the importance of vmcg is that it gives pilots time to decide whether to go or to abort the takeoff in case an engine fails after vmcg?
Vmcg is a certification thing which is used in the determination of numbers to be used by pilots. I suggest, presuming you are early in your career, that you don't worry too much about Vmcg at this stage.