Life steal and (perhaps) criticals don't stack, it randomly selects one or the other to use. For example, having an 8% and 12% life steal items will result in you stealing either 8 OR 12, not 20 (assuming health of 100).
I've never seen a critical stack either, though the percentage chance as opposed to the persistence of life steal means its harder to verify.
Go into a game against an Ai with Filthy rich, buy an Ashkandor and Slayers Wraps (10% for 4x and 10% for 2x Critical), then play for a while, you'll see a distinct lowish critical (600ish) a middle critical (1200ish) then very occasionally a combined critical of (1800+ish). Criticals do indeed stack if they occur at the same time, however this doesn't occur very often and the 2x or 4x is far more likely than the combined.
Life Steal also stacks, try the same only with Ashkandor and Mage Slayer, 12% early on is around 30-35 life steal, get both and you have 70+ life steal. Yes both items give you weapon damage which would increase your life steal but not by the visible amount.