May one say an after blessing if he vomits the food he ate?
If one vomited the food h ate prior to reciting an after blessing, then if he assesses that a Kezayis of the food has remained in his stomach, he is to say the after blessing. If however he is in doubt, then an after blessing is not to be recited. If possible one is to eat more food and then say an after blessing, or is to think the blessing in his mind.
 Ketzos Hashulchan 44 footnote 3; Ben Ish Chaiy Chukas 13; Kaf Hachaim 184/34; Neziros Shimshon 184; Torah Leshma 125; Shevet Halevi 1/208; See Piskeiy Teshuvos 184/8
Other opinions: Some Poskim rule one may recite an after blessing even if he vomited his meal, as we follow the benefit of the mouth that was already received. [Kol Eliyahu 9, brought in Kaf Hachaim ibid; Vetzrauch Iyun on Piskeiy teshuvos ibid who seems to rule exactly like his opinion, although negates it in the footnotes.]
 The reason: As by blessings we follow the benefit of the stomach and not the benefit of one’s mouth or throat. [Poskim ibid; Admur Seder 8/11; Luach 2/12; Shulchan Aruch 210/2]
 Ben Ish Chaiy ibid; Kaf Hachaim ibid