I struggled for a while with this yesterday. Even finding the user created stadiums isn’t easy.
Go into Stadium Creator Cycle through the “template” created stadiums, and on any you like, go to the right of the menu to save that stadium (you don’t need to make any changes first, but you can if you want). wait for what feels like 20 seconds (even when servers are good), then a pop up will ask you if you want to save that stadium to your vault.Until you do that, your created stadium vault is empty.
From the creator menu, you can also see stadiums that other people created, but don’t bother unless you have like a week to spare. It slowly loads like 8-10 at a time, there is like 100,000 of them with no rating or popularity filter, and most of them are just the template stadiums with a new name, or something really ugly. I hope they can find a way to highlight the truly good ones in the future, because if this feature relies on my own skills to make one, then it’s truly doomed.