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On a broader side, I started off with Theravada, which became my foundation for my move towards Mahayana and Vajrayana (at least I consider it being part of Mahayana) thus forming this One Vehicle like a tripod,
The Three Vehicles 三乗 {sanjo} or triyana of the Lotus Sutra does not refer to hinayana, mahayana, & vajrayana. Those designations were formed later on. It refers to the three vehicles taught by the Buddha:
Sharavakia, Shravaka, 声聞 {shomon}: Those who enter the Dharma Stream via a Dharma Gate revealed by a SamaSambuddha, Samyaksambuddha; 三耶三佛. One who completes this path, by becoming an arahant; arhat, 阿羅漢 is then known as an Anubuddha 所知 {shochi}.
Pacceka; Pratyeka, 縁覚 {engaku}: This refers to one who enters the Dharma Stream without the aid of a Dharma Dispensation by a Samyamsambuddha. One who attains arhat this way is called a pratyekabuddha. 辟支佛.
Bodhisatta; Bodhisattva; 菩薩: This is the same as pratyeka in that there is no teacher. It is different in that the Bodhisatta seeks the salvation of all beings; and thus acquires more merit. After attaining arhat, the Bodhisattva appears in the world as a SamaSambuddha, Samyaksambuddha; 三耶三佛 and reveals a Dharma Dispensation. Shakyamuni is the only current Samyaksambuddha for our world; he is our Treasure of Buddha at the present time. Nichiren stresses this point.
In my opinion, informed by study, the Lotus Sutra accepts these traditional three vehicles as Conventional or Relative Truth. If we wish to understand the Lotus Sutra on its own terms, this is what is meant by Three Vehicles. Then, we can begin to understand what is the One Vehicle. For one thing, the One Vehicle; Ekayana; 一乗 {ichijo} is what the Three Vehicles share in common.
In some of the 'pre-Lotus' Mahayana Sutras, the Enlightenment od the Anubuddhas is disparaged as inferior. The Vimalakirti Sutra confuses Samyamsambuddha with Enlightenment itself. I think this is done intentionally, with a sense of irony, to make us ponder what is Enlightenment? The Vimalakirti Sutra also confuses, deliberately I think, the distinction between Relative or Conventional Truth and Absolute Truth. Shariputra becomes a fictional character who represents this confusion.
In the Lotus Sutra, the Awakening or Nirvana of the Shravaka, Pratyeka, and Bodhisattva is declared to be equal, it is all anuttara samyak-sambodhi 阿耨多羅三藐三菩提. This awakening is available to all, in our present form, via adhimukti 信解 {Shingei} or 勝解 {shoge}. This is called 即身成仏 {soku shin jobutsu} or Enlightenment in one's present body. This is dramatized by the parable of the Dragon King's Daughter.
I do not think that the Lotus Sutra rejects the specific meanings of the Three Vehicles, but it goes beyond those by deriving general concepts from the same principles. It then uses various parables to depict and dramatize the source principles, general concepts,. and specific methods. It is also reveals, using visual imagery, the 4 inherent virtues of unbinding, as well various merits acquired through practice / cultivation. The specific {now shifting perspective} blessing of the practice of the Lotus Sutra is to awaken the innate virtues we already possess, represented by the Bodhisattvas from Underground. The general blessings are to gradually acquire merits represented by Bodhisattva like Monju.
Power of Fours; Qualities of Bondage and Unbinding