I have recently began playing again, and with an undead heavy campaign we needed to find a place for the rogue.
So we began looking at the game mechanics, first on Critical hits.
We all agreed that a Critical hit won't apply to undead......obviously.
We then looked at the other features of the rogue, the crippling strike. Now this can be used against undead, logically. Slice'n Dice the legs of an undead and it really can't move that well nor fight. It won't kill the undead but it sure puts it out of the equation for a while.
This worked well, but we found flaws, so we tried something new. We decided that when it comes to Criticals and undead, we only allow a Critical on a confirmed roll twice (meaning you confirm the crit) then ONLY if you confirm it again can you apply it to undead in the following way. We decided that due to the undead's lack of anything worth striking, on a basic hit location table the 2nd confirmation of a Critical allows not a Critical strike, but a MAX base weapon damage strike to a hit location with the effect of possibly damaging the specific location. Due to this being a type of "aimed attack" we added a -5 penalty to the attack roll but not to the confirmation rolls.
To us at least this worked fine.
When we looked at sneak attacks we did it the following way. Sneak attack, it means just that, you sneak up on someone and attack on a blind spot. Now there is no reason why you can't sneak up on an undead. The move silently as well as the hide skill both allow you to sneak past them. So with this in mind we decided again that with no "vital organs" to strike, we can still do a hefty job when blindsiding an enemy. A Vampire is an intelligent greater undead capable of learning new skills etc. and there is no reason he is immune to being sneaked up on so again the skill works.
So how to solve the added damage? We decided to make the sneak attack feature into a combo, where all the rules for sneak attack apply, however on undead (we never debated constructs, plants, etc.) we decided to allow a crit chance on the successful sneak attack. If the crit was confirmed only max damage was dealt, this is because you strike from the blind spot. You can't hit any vital organs but you can cause extra damage, though not any pain.
That is what my group worked out to work around the immunities. Alternate is a class from the path of the sword from Fantasy Flight Games which has a PrC, which is easily accessed as a monk but any class with the required feats can choose it. It is basically a monk with Divine Powered fists causing a lot of harm to undead and he has some nifty abilities, though not sneak attack.
An alternative is using the Monk then adding the ability to turn undead if sacrificing the fortitude save to equal that of a rogue. Also, this ability does not allow the character to take any other turning feats, unless given as a class feature. So the monk can never take extra turning, quicken turning etc, unless such feats are given by a class .