If you use standard design software like HTRI, HTFS, B-jac then FIV analysis is automatically performed along with Thermal analysis.
I think both thermal as well as FIV analysis shall be performed for all cases as both are interdependent.
please check if you can take advantage of ASME Sec VIII Div.1 appendix 32. It permits LTA (Local Thin area) in cylindrical shell for vessels.
What is the baisc cause of corrosion? One need to know this before recommending the solution.
Check with your dished end manufacturer and get this re-pressed again to 3400mm diameter. Other option of making to shell ID to 3476 will call for approval from client/process, etc. I suppose 3400mm is correct diameter.
What are the design conditions which are causing disned end of 25mm...