There are a couple things you can try, but it requires some knowledge of editing Vassal modules and some luck. As most people know, a lot of stuff that you add or change in a module does not transfer over to saved games. However, some stuff does get transferred, and you just have to know what does and how we can use that. I’m going to discuss two approaches. As far as I know, there is no quickie key-combo that will save the day.
(1) Edit the module. Add an “Inventory” function, which is a top level feature. Make sure all check boxes are on, which is the default. Save the module and exit.
Next, run the new module. Load the saved game that has the lost card or piece. You will see the Inventory function in the toobar. Select it. You then should see a list of all pieces in the game. Find the missing card, and right-click it. IF – a big if it has a command to send it someplace like a discard pile, deck, or dead pile. execute that command! It should send the missing card someplace where you can fetch it.
Once you got it,save the game, and pass it along to everyone. Everyone including you can use the original module. You can delete the one with the Inventory if you like.
(2) The second method is a bit more involved, but I’ve use it in the past to save the day, and it appears almost magical when it works. In this method, we give up on the missing piece and create a duplicate that replaces the original.
Edit the module. Find the piece wherever the module has it. Copy the piece to the Piece Palette. If the palette is disabled, enable it, or if absolutely necessary, create a new piece palette and put the missing piece there. (I’ve even added new images and created new pieces at this point.) Save the module and exit.
Run the new module and the saved game. Open the piece palette and grab the missing piece off the palette. Put it somewhere useful, and save the game. Give it to everyone. Everyone can use the original module, including you. You can delete the “fixup” module.
This method, btw, has more extensive usage. Imagine you’ve started a game with a hundred pieces on board, and every piece by accident is missing a key right-click command such control-F to flip it over. You might think this is a lost cause, but it can be fixed–easily if all hundred pieces are in a Piece Palette. Edit the module, and change all the pieces in the piece palette. Save the module and exit. Run the module, and load the flawed saved game. All pieces on the map will be wrong, but all pieces in the piece palette will be correct. Drag each corrected piece off the palette, and replace on map the old wrong one with the new right one. Delete the bad ones if you can, or put them somewhere out of the way–of the map edges will work.