MachyDude, a simple approach: Calculate the sesimic load per the relevant standard, IBC in your case, treat the vessel as a rigid body, analyze the supports for bending in the weak direction, analyze for overturning, analyze anchor bolt shear.
If your vessel shell is "large" and/or "thin" or has other unusual features, extra analysis may be in order.
MachyDude, sorry I don't have any building codes at hand. Often there are formulas for "non-building structures" or "mechanical components", etc. You are going to have to read the applicable code and make your best decision I'm afraid.
I was thinking IBC 2006 referenced ASCE 7-05, and it includes provisions for saddle-mounted pressure vessels, although it can take a while to sift through it all.
JStephen: Yeah, I was reading, seaching in both coads...but cudn't find any thing specific for saddle mounted vessel.
Finding somthing in these codes becomes real pain some time...