There is an easier way to have your cards automatically line up with a card row and that is to impose an irregular grid with a Region for each of the card origin points. The user will place a card on or to the left of the pile and the card will automatically snap on to the stack and for the row for that group of cards. This avoids the need for movement trigger and send to location.
However, there seems to be a bug in Vassal (v3.5/3.6 beta at least) that defeats this expected behaviour. If you find that pieces in a Player Hand are not stacking / displaying as you expect, it may be due to that issue.