PS-- I think you'll still have trouble collecting the skyshards which are in the enemies' base camps, but if you make Vengeance your home campaign and check the map often throughout the day, you might be able to go grab them when your alliance has gated the other two. As with any Cyrodiil campaign I've ever played in, the map tends to change colors throughout the day, so the trick is to determine what time of day your alliance is the one which has taken over the map, then be ready to go grab the elusive skyshards at that time.