Why did Samson | שִׁמְשׁוֹן Shimshon not observe the Vow of Naziriteship?
Nazir 4b 
A nazirite like Samson is permitted to become impure from a corpse ab initio, as we find with Samson that he became impure. Rabbi Shimon says: One who says he will be a nazirite like Samson has not said anything, since we do not find with Samson that an utterance of a vow of naziriteship left his mouth. Samson never took a vow to be a nazirite. He received his status from the angel’s instructions to his mother (see Judges 13:5). Consequently, Rabbi Shimon holds that one who vows to be a nazirite like Samson is not considered to have taken a nazirite vow.
Nazir 4b  states
וְשִׁמְשׁוֹן לָאו נָזִיר הֲוָה וְהָכְתִיב כִּי נְזִיר אֱלֹהִים יִהְיֶה הַנַּעַר מִן הַבֶּטֶן הָתָם מַלְאָךְ הוּא דְּקָאָמַר " The Gemara challenges the assumption that Samson’s naziriteship was not accepted through a vow: And was Samson not a nazirite whose naziriteship was accepted by a vow? Isn’t it written: “For the child shall be a nazirite of God from the womb” (Judges 13:5)? The Gemara answers: There it was the angel who spoke. Samson’s nazirite status did not stem from a vow uttered by a human being. "
Did Samon | שִׁמְשׁוֹן Shimshon become impure from battle at לֶ֔חִי Lechi?
"The Gemara asks: And from where do we derive that Samson became impure from corpses? If we say it is from the fact that it is written: “And Samson said: With the jawbone of an ass, I smote a thousand men” (Judges 15:16), perhaps he thrust the jawbone at them but did not touch them, and he remained pure." - Nazir 4b  : https://www.sefaria.org/Nazir.4b.18?with=Commentary&lang=bi
In Shoftim (Judges) 13:3-7, we learn the Angel of YHVH (מַלְאַ֨ךְ יְהֹוַ֜ה) and the Wife (הָאִשָּׁ֗ה) of Manoah (מָנ֑וֹחַ) make the Nazirite vow for Manoah's son Samson (Shimshon , שִׁמְשׁוֹן) to hopefully become a Nazir. * Samson never makes a Nazir (נָּזִ֗יר) vow. Samson prays and makes a request after he is humbled, losing his hair and sight.
Shimshon's Prayer in [Shoftim (Judges) 16:28]: "And Samson called to the Lord and said, "God YHVH, remember me and strengthen me now, only this once my Lord YHVH, that I may be avenged the vengeance for one of my two eyes from the Philistines." ( וַיִּקְרָ֥א שִׁמְשׁ֛וֹן אֶל־יְהֹוָ֖ה וַיֹּאמַ֑ר אֲדֹנָ֣י יֱהֹוִֹ֡ה זָכְרֵ֣נִי נָא֩ וְחַזְּקֵ֨נִי נָ֜א אַ֣ךְ הַפַּ֚עַם הַזֶּה֙ הָאֱלֹהִ֔ים וְאִנָּקְמָ֧ה נְקַם־אַחַ֛ת מִשְּׁתֵ֥י עֵינַ֖י מִפְּלִשְׁתִּֽים)
Samson never vows to abstain from wine (Yayin, יַ֣יִן), but His mother is told to abstain from Yayin during her pregnancy. * Children do not always become what their parents want them to become. Regarding the Nazirite vow in Bamidbar (Numbers) 6:2-3, a mother cannot make a Nazirite vow for her son. The vow must be made by a Yisraelite man or woman who sets themself apart to become holy for God.