I have a basic confusion about how to compute string scattering amplitudes in light cone gauge, using the operator formalism reviewed in GSW. I am familiar with the covariant gauge formalism, where I know how to use the state-operator correspondence to find the vertex operators corresponding to a physical state, but I'm not sure how to carry this over to light cone gauge.

To be specific, take the open string state $\alpha_{-n}^i|0,k\rangle$. A naive guess for the corresponding vertex operator is $\partial^n X^i e^{ik\cdot X}$. Is this right? If so, what is the relation between this operator and the DDF operator $A^i_n$? As far as I understand the DDF operator is supposed to be the covariant gauge realization of a light cone gauge state. Does this mean that the DDF operator $A^i_n$ reduces to $\partial^n X^i E^{ik\cdot X}$ if one fixes light cone gauge?

The main quantity I am trying to compute is the scattering amplitude between three tachyons and a very massive state. If anyone knows a reference that does this in detail please let me know.