So I did solve it, sort of 
The receiving deck must be set to always reshuffle. I put a dummy piece into the sending deck at the top of the list with my 2 textlabels (one for the name of the deck, another to count pieces into the deck minus the dummy pieces) the normal pieces go beneath this dummy file. The dummy file gets the same (coloured) image as the normal pieces and a does not stack normally/never. Apparently vassal sends the gamepieces from the bottom first to another deck, so my dummy goes as last in the deck and shows on top Because it’s not movable but selectable I can drag the pieces underneath out of the stack. As a bonus I can still use a dummy only for the black & white image in the receiving deck and the error from this post https://forum.vassalengine.org/t/calculated-property-gives-error-but-works/5186/1 only shows up in the editor, not during gameplay.
Stil got 2 side effects: The least important is that sometimes, because the deck must be set to always reshuffle the dummy is given to the player to be dragged out and it gets ‘worse’ with less pieces in the deck to shuffle, but retrying to drag a piece out gives eventually all the normal pieces to the player.
The second is when a piece is dragged back into the deck, that last piece is shown on top of the deck and gone are my textlabels. So a solution might be: give the dummy piece a marker=dummy and whenever the deck receives a not-dummy piece by drag&drop, send the dummy offboard to a deck and back in, but this implementation goes way over my head. If somenone can tell me if this is posslible and how, please do tell