Once again Harry only became Mab’s Vassal in Changes in taking on the Winter Knight Mantle, prior to that he owed Mab favours (through Lea) meaning he was not a vassal and not owed any form of obligation of protection by Mab. Mab was not Nicky’s vassal when she owed him a favour, nor is she Butters (and through him The White God) because she owes him a favour.
Vassallage is a bilateral relationship - Harry owes Mab, but Mab owes Harry. A favour is unilateral relationship, there is no corresponding obligation on the Person who is owed the favour. Mab took on all of Harry’s obligations when he was incapacitated, from tutoring Molly, to feeding the Za Guard to returning Harry’s overdue Library books which burnt in his basement in Changes (she obviously reconstituted them from the ashes handed them in and paid the fine) because he was her vassal.
The Fae can only apparently interact with mortals through a bargain i.e. an exercise of free will by the mortal. This would suggest the mortal has not to be under a disability, e.g. of age of sound mind and not intoxicated. Harry was still a minor in calling upon Lea for the first time he could not make a bargain with her. Ergo the bargain was with his mother to protect him. That was the first time (in file) Harry was physically threatened that we are aware of. The next time is in Restoration of Faith and there is no Lea to protect Harry. Harry is of age at this point suggesting his mothers bargain had expired by effluxion of time. Lea at this point is trying to inveigle Harry into his own bargain, which he stupidly does in GP, only to pull the Mushroom trick.
Harry has had the benefit of his bargain with Lea by being healed by her, his reneging on it and commuting it to ‘favours’ she has no obligation to Harry, it is he who has an obligation to Lea. Harry could die at any time with no progeny leaving his ‘favours’ unfulfilled, leaving Lea with a bad debt, but Lea has no corresponding obligation to protect Harry to do so without invitation is (1) meddling in mortal affairs and (2) leaves her with an unresolved gift, as Harry will not give anything in return . The only thing that Lea can do to protect her investment Harry has to be purely in the NeverNever so she creates the Murder Garden, not only to protect Harry but to spy on him and perhaps influence him in ways consistent with his Mothers bargain i.e beget Maggie.
Mab has given Harry a gift (Christmas Eve) but that is a boon to her Vassal who performed above expectations that year at the height of her powers a reward rather than a first. However no such vassalage relationship exists between Lea and Harry.
What I am trying to do is puzzle out the interactions applying between Fae and mortals, which by its nature HAS to be consistent with the exception of the nemfected. Quoting from the text is of limited value because it is based on Harry as an unreliable narrator with him having in the early books a limited understanding of the different relationships of favours, obligations, Vassallage etc. It should be noted that the Mantle includes an automatic adherence to Winter Law, meaning that Harry pre Changes point of view of these factors is very different to that of post changes he ‘knows’ things he previously only guessed at. The same facts looked at post-Changes may be assessed very differently by Harry.