A mortal has only one lifetime to attempt to outsmart and out-organize the immortal. The immortal has had several, if not several dozen, lifetimes to plan ahead, organize and make sure that doesn't happen.
While I agree with the general idea of vampires being smarter and having more experience than humans, there is something to be said about the overall implausibility of detailed long-term plans. In a state of flux you have to constantly change and adopt. And we're talking about vampires competing against among themselves. Under such circumstances it's not like someone is having the upper hand here. Rather, they are keeping each other in check throughout history. Because otherwise the conflict within the vampire race would be long over by now, which certainly isn't the case. But it's useful fasade for the top vampires to keep, so I imagine they would help to sustain such impression in the general vampire populace as much as possible.
The problem isnt their plans failing, problem is Deus Ex Machina, you have a character that is depicted as a mastermind sort of character, out of sudden acting like a tard just to fit your story, that means you are a lazy writer, the story wouldnt proceed the way you wanted if you didnt bend it backwards to do it so, the hero would die if the gods didnt save him, those half assed tricks are transparent and break suspension of disbelief hard as you can easily see the hand of the writer covering his ass . Good stories have an internal logic and the writer cant destroy the logic he created under his convenience or you stop buying his crap.
If a 500 years old vampire is killed on a really well planned hit organized in such a way to deal with him on a way to neutralize his strengths is something nice to read, but just reading a guy acquiring a terminal case of retardation feels like reading half assed schlock. Unfortunately, making good stories is hard so you have convenient cases of terminal retardation everywhere.