The first step is incorrect:
- You take electrically neutral cyanoethylene, but on your mechanism it is negatively charged. So you create negative charge of nothing on zeroth step of the mechanism. The same is for positive charge on enamine
- You cannot pluck the proton from enamine in this way.
At the first step the electrons of π-orbital of enamine interact with the π*-orbital of acrylonitrile(Michael acceptor) resulting in formation of the bond between alpha-carbon of enamine and beta-carbon of acrylonitrile.
The transformation of iminium to carbonyl occurs via tetrahedral intermediate and not via formation of vinylic carbocation.