Pomacea diffusa isn't a species that typically eat plants but they obviously create a lot of waste. I keep them in nearly all of my planted tanks with no issues.
The substrate change could definitely be your issue but it could also be that the shrimp were recently shipped to you. No matter the species, shipping is stressful. Especially if they were recently imported. (This is why I don't like to buy shrimp that haven't been allowed to settle in for 3-4 weeks before they get shipped to me. Just a risk I don't like to take.)
And definitely don't vacuum your substrate in a shrimp tank. I wouldn't even try cleaning the surface of it unless it's to remove large chunks of uneaten food. And even then, I'd just use a piece of airline tubing and do it quickly without disturbing the substrate too much.