Forgot to say Vesein Tal Umatar:
One who forgot to say Vesein Tal Umatar during the period that it is to be said, which is in Eretz Yisrael between the 7th of Marcheshven and Pesach, and in the Diaspora between the 5th/6th of December and Pesach, the following is the ruling: [If one remembered prior to saying Hashem’s name in Bareich Aleinu, then he is to say it in the place of remembrance]. If one remembered after completing the blessing of Bareich Aleinu, but prior to beginning [the first word of] Teka Beshofar, he is to say it there. If one already began saying Teka Beshofar, he is not required to return to Barecih Aleinu and rather is to say it in the blessing of Shomeia Tefila [prior to “Ki Ata Shomeia”]. The same applies if he remembered anywhere between Teka Beshofar and Shema Koleinu. If one remembered after concluding the blessing of Shema Koleinu but prior to Ritzei, he is to say Vesien Tal Umatar and then begin Ritzie. If one remembered after beginning Ritzei but prior to taking steps back at the conclusion of Shemoneh Esrei, he is to return to Bareich Aleinu. If he already [completed Shemoneh Esrei and] took three steps back, he must return to the beginning of Shemoneh Esrei. Furthermore, even if he did not yet take three steps back but has already recited the verse of Yihyu Leratzon at the end of Shemoneh Esrei, he must return to the beginning.
If one is in doubt as to if he said Vesein Tal Umatar:
If one is in doubt as to whether he recited Veisein Tal Umatar then if he is within the 30 days from when it was begun to be said [within 30 days after the 7th of Cheshvan in Israel, and within thirty days after the 5th of December in the Diaspora] we assume that he did not say it, and it is to thus follows the same law as one who omitted it. After the passing of thirty days, we assume that he said it, and hence he is not to go back and repeat it.
Chart for one who remembered that he did not say Vesein Tal Umatar:
Area of Shemoneh Esrei
Prior to Hashem’s name in Bareich Aleinu
Say it there
Prior to Teka Beshofar
Say it there
Prior to Hashem’s name in Shema Koleinu
Say it there
After saying Ritzei
Go back to back to Bareich Aleinu
After saying 2nd Yehi Ratzon
Repeat Shemoneh Esrei.
 117/5; Michaber 117/5; Brachos 29
 See 114/5
 Admur ibid; M”A 117/5
The reason: By saying it in this area it is considered as if he is saying it in Birchas Hashanim, as explained in 114/7. [ibid] There it is explained that so long as one has not yet begun the next blessing the previous blessing is not considered to have been concluded regarding these additions that must be recited, and invalidate the prayer if omitted. However regarding those additions that do not invalidate the prayer when omitted, the conclusion of the previous blessing ends that blessing even prior to beginning the next blessing, and hence one may no longer recite the non-invalidating additions once one has completed the blessing. [114/7]
 Admur ibid; Michaber ibid; Gemara ibid
 Admur ibid; Michaber ibid
By saying it in this area it is considered as if he is saying it in Shomeia Tefila [ibid], as explained in the previous footnote.
 Admur 114/10 regarding Mashiv Haruach and the same applies for Visein Tal Umatar Levracha [Kaf Hachaim 117/32]
 The reason: As until the pass of 30 days [90 prayers] of the recital of the new addition we assume one said the usual wording that he was accustomed to say until that point, unless he knows for certain that he recited the correct dialect. [ibid]
Reciting it 90x: Prior to the conclusion of the 30 days one is not to repeat the beginning of the blessing of Michayeh Meism until … Morid Hageshem for a total of 90x in order to make for oneself a status quo that he has now regulated himself to say it and hence no longer needs to go back in case of doubt, as there are opinions which argue on this ruling and hence one ends up entering himself into a dispute if he does not remember if he said it prior to the 30 days concluding. [Admur 114/11]
 If he did not Daven some of the days within the 30 days: The above law applies even if there were a few days in-between in which he did not say it, such as he did not Daven at all, his Chazaka is not weakened, being that he said it for majority of the days. [Admur ibid]