Sounds like in this case, "most stringent code" just means they don't know what code it falls under, either.
Typically, in the US, the vessel itself would be designed per the ASME Boiler and Pressure Vessel Code, assuming the size and application fall under that code. Refer to the various pressure vessel handbooks for attachments to the vessel, which would typically be a saddle-type attachment with analysis based on L. P. Zick's work from long ago. Design loadings for the support structure would be taken from ASCE 7, which unfortunately leaves a lot to be desired when used in certain applications. The steel structure itself would be designed per AISC steel codes. If the vessel is supported by the building, you'd need to consider interaction with the building for seismic loading. You may also have a local building code, which usually will reference one of the standard building codes, and may or may not address anything specifically related to your project. (Around here, the local public libraries will have reference copies of the version of the building code used in their city.)
