After passing the test of the akeidah Avraham is told his reward: "V'hisbarchu b'zaracha kol goyei ha'aretz eikev asher shamata b'koli." (22:18)
Some of the meforshim ask: what kind of a reward is this? Avraham had already been promised this by G-d at the beginning of Lech Lecha: "V'nivrechu becha kol mishp'chos ha'adamah." (12:3) Now, ten tests later, he is promised the same thing all over again -- that's it?!
I want to suggest a solution based on a beautiful Netziv elsewhere in our parsha. B'pashtus, the story of the angels being welcomed by Lot and being invited into his home in contrast to the attack upon them by the rest of the populace of Sdom serves to establish a zechus for Lot so that he would merit being saved and seals the fate of the people of Sdom by their wickedness. Yet that can't be. As we discussed last post, the fate of Sdom was already sealed before the malachim even got there. Hashem had already refused Avraham's prayers on their behalf and one the angels was on a mission to destroy the city. The other angel that came to Sdom was there to save Lot, so his fate too was already sealed. The reaction of Lot and the reaction of the people of Sdom to the arrival of the angels did not make any difference to the outcome. So why is this story interjected?
Netziv (Harchev Davar) answers that there is something even better than G-d given you a free favor and rescuing you in a time of need. What's even better is earning that gift and reward. G-d would certainly have saved Lot no matter what; Sdom would have been destroyed in any case. However, by sending the angels, Hashem provided Lot with the opportunity to do chessed and earn the rescue that he was going to be given (see also Ohr haChaim 19:1). Hashem have the people of Sdom the opportunity to demonstrate their wickedness so that there was no question of their deserving everything they got.
The greatest gift that Hashem can give is the opportunity to serve him and earn the bracha and yeshu'a that is needed and not have to get it as a handout.
Perhaps the key difference between the bracha in parshas Lech Lecha and the bracha in VaYeira is the end of the pasuk: "Eikev asher shamata b'koli." EIkev means you've earned it; it's not a favor, but rather it's a justified outcome. That could only come after the tenth test.