Can we use CAESAR II model full/half vacuum system?

Not really. In a typical piping flexibility analysis, pressure is assumed not to cause displacement of the piping system (ignoring the effects of expansion joints). Pressure is accounted for at the end, in the stress computation, in the form of the longitudinal pressure stress component of the code equations. In its simple form, this term is computed from PD/4t and added to the stress.

The 3D Beam Element used in piping programs is an infinitely thin stick. It only knows about displacements of its end nodes. You can't evaluate buckling, collapse, or other local effects, because the formulation of this element is unaware of these phenomena.