Do you mean for cloth diapers? Then the answer is probably not, although it sounds like an interesting test. I've seen vinyl bloomers, but never tried one. They do have leg cuffs just farther down. Waterproof pants aren't supposed to be water tight around the legs. Just waterproof enough to not let moisture get through to your clothing. Still it probably won't work because the wetness might seep down your leg. Try it and report back.
Just a fair warning, putting plastic pants over disposables doesn't help increase capacity and will only help very little. It will help you feel wet when the diaper is leaking and help warn you. Yes it helps but it won't do anything with the liquid. It won't make any diaper better.
You don't need to purchase anything else if you do buy plastic pants. They usually have their own leg gatherers (not leg guards). They're usually not waterproof.
Plastic pants are used as a water proof barrier for cloth diapers. Just as disposable diapers have a waterproof barrier built in it.
Huh? I guess you're not talking about cloth diapers but disposables with a cloth-like cover. Let's got with that.
Waterproof pants of any kind will catch leaks, but of course they won't absorb them. That means that even the best-fitting ones will only hang on to the urine until you take the pants off. Then the wayward urine will have to be dealt with. The best thing to do is to wear something absorbent over your diaper--preferably something that is also snug enough to hold the diaper in place as it expands and gets heavier--and then put a waterproof pant on top of that. This way, any leaks will be absorbed and won't just roll around in the pant looking for a gap to leak out of. This combination will offer exceptional protection and allow you to push the limits of any diaper.