WebDiplomacy also has a sandbox, which is the one I use commonly. It supports 9 variants, and allows for custom country starting SC’s in all the maps (except maybe chaos).

Bump

I tend to use the Politics Order Judge as a sandbox to figure out complicated phases on the classic map:
http://gamesbyemail.com/Games/Politics/Judge

Unfortunately, using it as a sandbox requires dragging and dropping a cursor, so the website is only really usable as a sandbox on a computer, not mobile. To get started, click on the "Clear board" hyperlink under "Other:" and drag and drop the units you want into place.

For Android users playing a classic game, Little Dipper can still be useful and should be familiar to vDip users since it's built off the same phpDip/webDip code:
https://www.getjar.com/categories/all-games/board-games/Little-Dipper-388543

Unfortunately, the app relies on the long-deprecated Android menu key to access key features like editing the board to fix mistakes, so you either need an ancient Android device handy or to rely on a third-party app to reenable to menu key on newer devices. I got it to work with "Menu Button (No root)", by Jetpof Apps, but ended up uninstalling since I felt uncomfortable with all the ads and required permissions.

Bumping over the spam