Page 184 of pathfinder CRB it talks about how you can crit with spells that require an attack (i.e. vampiric touch) so if I had a weapon of storing (or more accurately a custom weapon that cast vampiric touch on each hit) if it crits would the spell also critWhich spell are we dealing with?
I can't think of any touch spells off the top of my head that work extra on crits.
On the primary question, yes I would say it crits when the weapon it is stored in crits. A critical represents striking a vital spot for extra damage, there is no reason it shouldn't do the same for the stored spell. I'm saying this without any official reference to rules, just my own interpretation if there are no rules on the subject.
Rules do have to bow to logic sometimes.![]()
If the stored spell would normally require an attack roll to hit than you still need to roll to hit when it is released. Any attack that dose Hp damage that requires an attack roll can crit. So you could, if you stored the correct spell and were very lucky, crit once on the weapon and once on the spell.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.