It would depend on the relationship between the liege and vassal. In some cases the vassal would expect the liege to help protect him, e.g. Tier being helped against the Spider. In other cases, where the vassal has more of an independent streak, such as with Bellamie, it would be more likely that the Liege stays out of the vassals way. In the end it will depend on what diplomatic agreement exists between the two parties.