Well, first it sounds like the shower curtains may be more of a hindrance than a help. Being slippery to walk on will make it hard for bunnies to walk, much less run around, so that may be part of the problem right there. Second, If you are cleaning with the stuff that they know to stay away from, ie. vinegar and nature’s miracle, you may not be rinsing it off enough to lose the smell, and they may be repelled by the litter box areas because they don’t like it.
It’s kind of hard to picture what you’re saying, but let me suggest this. Remove the curtains completely. Put a lot of litter into the pans, and make it unscented and good odor absorbing like Aspen shavings (do NOT use Pine shavings ever) or Feline Pine pellets, or Carefresh or Yesterday’s News litter brands. When you see either one go in the litter box, praise them immediately and give them a treat while they are using the pan, just one little raisin while they do their business.
This is positive reinforcement for using the litter pan. When they go outside, you can say a firm “No” but then ignore it, scoop up bunny and set back into the litter box immediately. That should associate going with litter pan, but don’t wait at all because the meaning will be lost. Then, in the litter pan, say “good bunny”. If you find the pee well after they’ve done it, just ignore it because it’s too late for them to understand. You want to make the litter pan a good place to be but not have them search for a new spot outside.
I don’t remember if they are spayed yet, but that will help since this also may be marking behavior.