Wow, just found this thread... ANYWAY, I just want to say that poop staining plastic pants will lead to their demise... Eventually, the fats and body oils in the stain will cause the the PVC (Vinyl) to break down, become crackely and easily torn. But the silver lining is that you get to buy more plastic pants! (Help keep our manufacturers in business!)
In almost 60 years of this silliness, I have not found a way to remove poop stains on vinyl. Poop changes the chemistry of the vinyl under it. Having a soaked wet diaper will keep the poo wet- to an extent, and if you turn on your shower, pull on rubber gloves, step in, take your ppp's off immediately, and run them under the shower, or the regular tub nozzle- (Under the nozzle will keep the poop from splashing all over better than the shower.) Wash with plenty of soap!