Another solution is hinted at in the short story, Something Borrowed. In that story, Bob says that Will marrying Jenny Greenteeth (disguised as Georgia) annuls Will's true love protection (and allow Jenny to enslave him). In his words:
“I mean, if he’s married to another, it can’t really be pure love. Jenny’s claim on him would prevent the kiss from lifting the spell.”
One could argue, based on this, that Harry marrying Lara would similarly annul his protection from Murphy. And conveniently, there's a wedding coming up. Now Jenny is a Sidhe not a Vamp, so it might not be valid to analogise these scenarios. But it is definitely suggestive.