decode,
You are correct, you cannot use used parts to construct a "new" pressure vessel. Your interpretation comes from the ASME Code which only governs "new" construction.
Subject to Jurisdictional and AI acceptance, you can use an assembly as a replacement part to repair/alter a vessel. See the following interpretation:
Interpretation: (NBIC) 95-15
Subject: R-307 Replacement of Pressure Parts, 1992 Edition with the 1994 Addenda
Question: Is it permissible to use an assembly from an in-service pressure vessel as a replacement part for the repair/alteration of a second vessel?
Reply: Yes, provided the intended repair/alteration has the concurrence of the jurisdiction and the Authorized Inspection Agency.
You can even use material from another vessel provided the material falls under the same Code of Construction and also subject to Jurisdictional and AI acceptance.
See the following interpretation:
Interpretation: (NBIC) 01-28
Subject: RC-1040, Materials, 2001 Edition with 2002 Addendum
Question: Is it prohibited to use material that has been previously in service for replacement material for a repair if that material otherwise conforms to the requirements of the original Code of Construction?
Reply: No, provided the use of the material has the concurrence of the Jurisdiction and the Authorized Inspection Agency.
I agree with metengr as far as ASME is concerned. There is only one pressure vessel with a stamp here. and should remain as one stamped vessel.
What is to prevent the R-Stamp from being applied to the other remaining part? I have been involved with R-Stamp repairs to other than ASME Stamped pressure retaining items. The real question is, which Code should be followed to satisfy the Jurisdiction, Insurance Company, and AI(AIA)?
Lots of questions come to mind...