The title comes from Sherlock's second speech in TSOT, after he finishes playing the waltz:
More importantly, however, today we saw two people make vows. I’ve never made a vow in my life, and after tonight I never will again. So, here in front of you all, my first and last vow. Mary and John: whatever it takes, whatever happens, from now on I swear I will always be there, always, for all three of you.
So the last vow is Sherlock promising to take care of John and his family, no matter the cost.
Mary
Last edited by maryagrawatson (August 31, 2014 5:39 pm)