I'd caveat that with the conditions that we are currently seeing his performances under a particular set of technical regulations coupled to a particular qualifying format and memories that are somewhat biased towards more recent events (which is part of human nature).
Weighing things up, if you were to go back, say, five to ten years and ask the question "Who is the best qualifying driver on the grid?", you'd probably be pointed towards somebody like Jarno Trulli (especially in the wake of his front row grid slot for Jordan in Monaco back in 2000), yet these days Heikki has easily had the measure of Jarno when it comes to qualifying. Similarly, if you were to go back to, say, 2003-2006, others might point you towards Mark Webber during his days with Jaguar, and also Williams to a certain extent, when many felt that his qualifying performances flattered the car, such as when he qualified 3rd at the 2003 Hungarian GP in the Jaguar R4, or qualifying in 2nd place in the 2004 Malaysian GP in the R5.
Vettel has certainly performed extremely well throughout qualifying this year with pole position after pole position, but on the other hand it should be borne in mind that qualifying performances are somewhat relative. For example, the F150th is know to perform relatively poorly over a single lap, and development on the car more or less stopped in Hungary - so, would you say that Alonso has not performed well in qualifying today, given that he is only in 5th place, or would you say that his performance today was excellent because he is close to Webber (within 0.2s), considering that Red Bull have continued developing their car for much longer (even though they have won both titles they are still introducing updates for the RB7, like a slightly revised front wing for the Indian GP).
How exactly do you judge that Vettel is, singularly amongst the drivers on the grid right now, doing the best job possible considering the relative form of the cars on the grid? Are Vettel's pole positions, for example, comparable to Heikki managing to pressure the tail end of the midfield for 17th at times this season in qualifying, considering that Heikki is driving for a team on a fraction of the budget of Red Bull?
Go further back in time still, and there are other instances of drivers who also stood out in qualifying, and arguably to an even greater extent that Vettel does now, such as Jim Clark, Fangio or Moss, all of whom at times would utterly dominate in qualifying or manage to put a car far further up the grid than many would have expected it to be. Right now, it is true that Vettel is doing very well in qualifying, but that is by no means assured over the full length of his career. Give it another, say, five years, and we could all be hailing another driver currently further down the grid, or currently still in a junior series, as the greatest qualifying driver the sport has seen - so, for now, I'd hold fire on declaring Vettel the greatest driver in qualifying in F1 history for now.