As I understand it you can wait until after your anniversary and then cancel during the first month of your new year even if your renewal fee hits your statement as long as you do not use the cards after the anniversary date. Assuming you check online you will be able to see when the voucher gets allocated to your VS account, it should a maximum of 30 days after your year end but is often quicker.

This is second hand information read on FT and on HFP but I am confident in its authenticity as I have seen/read it many times from people whose opinion I value based on previous comments.

I am 10 months into my VS black and only today heard back from MBNA confirming my calculations that I have spent over £10,000 on the Amex card (visa spend does not count of course). I will not use either card again and pay off the outstanding balance on this statement run (month 10) ensuring I can contact them in April/May just after my voucher (and renewal fee) hits.

Months 11 and 12 wont have any charges made to either card and whilst this is not strictly necessary in the context of "cancelling and getting the voucher" I have flipped my spend to BA chasing another 241 there.