This something I have been working on for a decade. In my view, it goes to the very core of the Lotus Sutra; the distinction between the provisional, trace gate, three vehicle teaching and the ultimate, source gate, one vehicle teaching. It is not difficult, but does get rather complicated to explain. For that reason, I intend to move slowly and try to avoid zigging and zagging too much. I want to start with some comments by Jan Nattier from “A Greater Awakening”; which was published in the Spring 2006 Tricycle Magazine.

The “greater vehicle” had, initially, one specific meaning. It was the vehicle, or path to awakening, of one intending to become a Buddha, which was the original meaning of the term bodhisattva. See: http://www.tricycle.com/special-section/greater-awakening

The Lotus Sutra was most likely composed in response to very early Mahayana. The early Mahayanists drew a distinction between the unprecedented /unparalleled / supreme (anutarra), complete (samyak) awakening (sambodhi) of the Buddha; and the incomplete or partial awakening of the arhat. Thus, they rejected the attained nirvana of the pratyeka and shravaka arhats, opting instead for a higher goal, that of becoming an Anutarra Samyak SamBuddha, like Shakyamuni.

They were not talking about Mahasattva Bodhisattvas, Buddha Nature, Emptiness, Immediate Awakening, Dharma Body, or One Vehicle. Those concepts had yet to be fully developed. They were taking about pursuing the Bodhisattva path as understood in the Agamas, that is, the path pursued by Shakyamuni / Gautama, when he still an unawakened Bodhisttava, or by the Coming Buddha, Maitreya; who, by inference, was considered to be the current, head of the line Bodhisattva.

In other words, the word Mahayana, in its original usage, did not comprise any new doctrinal content whatsoever. It meant nothing more and nothing less than “the bodhisattva vehicle.” — ibid

In that context, the Bodhisattva could not awaken in this lifetime or even the next. It would take countless lifetimes spanning many kalpas to earn sufficient merit and acquire enough wisdom to appear in the world as the Anuttarra Samyak SamBuddha.

Would-be bodhisattvas had to look forward to thousands, if not millions, of additional lives before Buddhahood could be attained. Further, it was assumed that in those lives they would perform the kind of extreme acts of self-sacrifice described in the jatakas, in which, for example, the Buddha-to-be, out of compassion, allows himself to be devoured by a hungry tigress and her cubs or to be cut to pieces by an evil king. — ibid

Moreover, there were special rules to be a Bodhisattva / Buddha-to-be.

… since the very definition of a [samyaksam] Buddha is someone who discovers the way to awakening by himself in a world that knows nothing of Buddhism, they could not become Buddhas here and now. Rather, that final step had to be reserved for another time and (in most cases) another world-system. So the aim of these pioneering bodhisattvas entailed “rediscovering” Buddhism for the benefit of all beings in the distant future, when the teachings of previous Buddhas had long since been forgotten. — ibid

The attainment of nirvana / arhatship was not seen as an intermediate goal, as a step between binding to karmic existence / samsara and full awakening. It was actually viewed as something to be avoided, since it would prevent one from ever becoming a full blown Buddha like Shakyamuni.

Given this scenario, the possibility of arhatship becomes, ironically, a threat. The early Mahayana scriptures still regarded its attainment as quite accessible even within this present lifetime. Meditation—especially the practice of the dhyanas (Pali, jhanas), or states of concentrative absorption—is viewed as a particular danger, since the budding bodhisattva may inadvertently “tumble into” arhatship. — ibid

iirc, in the Vilmalakirti Sutra, Shariputra and the other arhats actually express regret that they had attained nirvana and could never become Buddhas? Something like that.

In a future entry, I’ll try to explore the foundations of the Three Vehicle doctrine in the ‘early’, pre-Mahayana Buddhist teachings. Since I do not have sources for the Agama Sutras, I’ll use on line translations of the roughly equivalent Nikaya Suttas, plus some Theravadin commentaries, and so on. AFAIK, there is no explicit three vehicle doctrine in the Suttas. However, all three are mentioned by name in the Suttas, as well as some Theravada commentaries. Moreover, there is a parallel doctrine of three types of Buddhas. The three vehicles were, initially, just the respective paths for each type of Buddha.