Welding on a Stress Relieved Vessel

I think this requires a contractor with an ASME "R" stamp for field repair or modification of vessels, and also some type of in-the-field PWHT equivalent to any PWHT on the original vessel.

Rather than that approach, how about making a "riser clamp" that surrounds the vessel?

You can weld or making any required repair in a vessel which previously PWHT using qualified welding contractor, and that contractor must hold certificate of authorization R-stamp (by NB) only in case of that vessel was granted one of the following certificates of authorization U, U-2 or UM-stamp (by ASME) as a demand for reserving the warranty, guarantee and valid insurance.

And you can proceed a local PWHT for the welded portion only with applying the rules of code.

Localized PWHT for a girth weld of a pressure vessel

Like the other two answers, you need to be up on requirements for welding any pressure vessel. If by chance your not or the contractor is not, very bad things can happen, like what happened a few years ago in northern Indiana. Contractor welded a high pressure vessel and it then ruptured killing several people along with the welder. Contractor was not certified in these repairs. Need to look at what your doing with a careful eye. Asking questions such as yours, does prevent injuries. Good Job!