In the design of Pressure Vessel - Anchor bolts, should we take the allowable stress of Anchor Bolt material in Tension or Shear (for vessels mounted on skirt ?).

If you are using the PV Elite, check out the 'skirt' design in the calculation examples. You'll notice that you only nominate the bolt type and material, the rest is done by the program. Please note that the example is not quite correct, because you are supposed to use lower grade of bolting, not B7 (that is, use structural high strength bolting). Just to complement the above, the bolts are calculated for prevailing load, in tension, due to the vessel overturning loads.
cheers,
gr2vessels