Can legions in Cardfight!! Vanguard be broken/un-legioned?
Yes, although it can only happen as part of the effect of another card - it's not something you can choose to do when your vanguard is in Legion. For example, Star-vader, Blaster Joker can retire one of the opponent's legion mates.

In the case where Legion is broken, the vanguard is no longer considered to be in Legion, and it cannot Legion again. You must ride a new Legion Leader and activate Legion with it.