We have a potential study of an existing buried pipeline subject to vibration from tunnelling nearby.

We have done buried models before and also vibration but are unsure how caesar will handle the two.

Please advise if you have done this before and if any sensible stress results were derived. Our fallback position is to use FEA software to examine an element of the pipeline.

You need something much more sophisticated than a general purpose piping program. Basic 3D beam theory, with discrete restriants (simulating a continuous beam on an elastic foundation) has too many course approximations to yield reliable results in this situation. FEA is the way to go, assuming you can model (mesh) the soil also.