I have a vertical vessel made of outer 10" pipe about 12" high. The vessel drawing is marked Sect VIII Div 1. My connection to the vessel is a 2" pipe unreinforced connection. The question has come up as to whether the vessel nozzle should be qualified per B31.3 using an unreinforced tee connection and modeling the vessel as pipe or whether the loads at the shell of the vessel should be checked per WRC 107? Which method is recommended or are both methods acceptable?

B31.3 will only give you the requirements as far as "area replacement" go. This won't indicate whether or not you have overloaded the nozzle or vessel. So you also need WRC-107.