Bad transfers are one thing, transfers that set us back 5 years don't come every time. Vidic is one of those and possibly the worst of the lot based on that. We pretty much shot ourselves in the foot that summer and Vidic was the icing on the cake. With more prudent transfers and avoiding to get rid of players that we did not to get rid of (Cambiasso, Samuel in particular and of course sort of forcing Zanetti to retire), with just one quality addition we would have been far more relevant during the Thohir era.
We let Cambiasso go, replaced him with Medel for more money (8m + salary).
We let Samuel go, who didn't want that much money anyway, to get Vidic for a 5m (10m gross) salary.
Joao Mario/Gabriel Barbosa was also a disaster as we were penalized in Europe for a few years for that on top of everything else we endured before.