You're grappled, in the dark, and can't pin-point vital organs.
If you can't see the question is moot. So let's assume that the PC can see.
How on earth are you going to justify not being able to pinpoint vital organs?!?!? You are inside a vital organ! The entire area is a vital organ.
No where does it state that being grappled prevents you from sneak attacking. Rather, by exptrapolation, one who is in a grapple does not lose their dex bonus with respect to anyone else in the grapple and therefore usually does not get to SA their opponent.
But this is also irrelevant as the worm isn't even considered to be grappling so grappling is a nonissue.
Therefore, the only important part about being on the inside of the worm is that the rogue does not threaten an area, is surrounded by vital organs, and is invisible to the worm (as the worm certainly can't see inside its stomach). As the rogue's own space has vital organ areas he can target those vital organs and thereby get sneak attack (since he is considered invisible).
Again, assuming he can see, the entire area around the rogue is pretty darn vital to the worm. I don't see how you could justify not giving the rogue sneak attack?