V- for velocity vector | v |- for velocity (rate).
Synthetic acceleration is the first derivative of velocity vector with respect to time, that is, dv/dt. Not only the change of speed is considered here, but also the change of speed direction is considered.
The tangential acceleration is the first derivative of velocity with respect to time, that is, d|v|/dt. Only the change of speed is considered here.
The two cannot be confused.
For example, if the velocity vector v= 2ti +3j, then the velocity |v|=√(2t)? +9
The composite acceleration a= dv/dt = 2i is a constant.
And the tangential acceleration a 1=d|v|/dt = 4t/√(2t)? The size of +9 varies with time t and is not a constant.