I just got out of a custom game to test out Vayne's Tumble and I believe I have tumbled upon something(sorry for the bad pun :P). Originally as I tested Tumble, I kept my target even though I had used tumble.
However I notice this functionality changed while I used Tumble during Final Hour. It seems that when you use Tumble while Final Hour is active, the stealth portion of Tumble will actually make Vayne stop all action. This would make sense since if you would just immediately auto attack the stealth would be gone instantly. I believe this is a built in intentional functionality of Tumble's stealth mechanism.
It is also possible that you are not used to how Tumble works. You do not need to click on the map for Tumble to work. You will tumble towards your mouse. When I first started playing Vayne I had a bad habit of clicking where I wanted to land. This will indeed break focus from your target.