I am not sure exactly what part of the analysis you are having trouble with, but I would use ASCE (or the EU equivalent) to develop the wind and seismic loads. These loads can be added in manually if the software will not compute the loads to the standard you are wanting to use. WRC or FEA can be used to check the leg to shell connection, and then use the column support calculation from either Brownell and Young "Process Equipment Design" or design of support legs calculation from Jawad and Farr "Structural Analysis and Design of Process Equipment". Both references discuss base plate design.
The design in both books basically treats the system as a cantilevered beam. you develop base shear (transverse shear), bending moment, and tension/compression and choose a column section that meets the need. Once column loads are determined, AISC (or the Eu equivalent) can be used for column selection.
Pressure Vessel Design Manuel by Moss is also another good reference.
Also, for the loading, use the worse case of wind and seismic...but these are never applied at the same time. Also, if you have platforms and ladders going up the side of the entire vessel, or some percentage of it, I typically add 12in to the effective diameter to account for the ladder drag, and any piping you have that is running up the side make sure to add to the effective diameter.. most software will not add piping into the wind calc. Any piping on top of the vessel will need to be accounted for using a point load and the CG of the piping. Also, make sure the program is accounting for all the platforms in the wind calculation and the dead weight.
