She made the vow well after that though. Vow was during Sister III pt 5 (strip of 12 Oct 2016), Sarah found the mark at the end of Squirrel Prophet (strip of 13 Feb 2015). You can't vow to do something you've already done, and neither what she says in the 12 Oct strip, nor the one after, suggest that she thinks it'd be fulfilled. Remember, the key criterion for whether immortals can get away with stuff is if they can genuinely convince themselves that whatever they do is in line with their vows and immortal law.