I've never seen, heard, or read about any such wackiness in Section VIII, but I'm far from an expert. Is the flexible material to be the shell or a liner? What's the material? Many plastics fall under Section X. I believe there are also flexible/expandable vessels under ASME PVHO-1.
The quickest way to disqualify your "vessel" from the Section VIII, in my opinion, is through material requirements.
Skip the scope and read ahead to Part UG-Materials and descend the rabbit-hole. In a nutshell, only materials listed in ASME Section II and used in conjunction with Subsection C of Section VIII may be used. There are many exceptions and exemptions, many of which will be referenced in UG-4. What's your application, out of curiosity?