This discussion talks about different ways to model a sandwich structure.

This issue for viscoelastic materials is they are nonlinear and the harmonic response analysis is limited to linear materials so you will be forced to run simulations in Transient Structural and extract time history data and manually create the harmonic response that is automated in the harmonic response analysis.

Here is discussion one and two on someone doing a Transient Structural simulation because they need nonlinear geometry (Large Deflection).

Please reply with details about the viscoelastic sandwich, the kinds of structures you want to analyze, the range of frequencies you want to study.