I used 6.6.3 for two games recently. The first game had internet connections in the same city area. The second was from Montreal to rural Sweden. In the second game, we had a lot of trouble with getting out-of-synch. I or my opponent would use Ctrl+Click and the other person would not see it. Similarly, I or my opponent would move a unit and the other person would not see it. Even stranger, several times I could not see my own Ctrl+Click. When it happened, we would play around clicking and control clicking and after a minute or so, things would return to normal. After about an hour of this intermittent behaviour, we went back to 6.2.2 to continue our session. The behaviour did not happen with 6.2.2 . Skype was running fine during that time. The first game had no such problems. Weird stuff.
Both of us have VASSAL 3.5.8 .