I am an old student of S.N. Goenka Vipassana treat. I want to know whether Vipassana alone is sufficient to achieve Nirvana or not? Did Siddhartha Gautam became Buddha using Vipassana alone? Or If he employed some other techniques what were those?
It's reported that Gotama spent 6 years training all sorts of things before he achieved Nirvana. This training involved studying under other ascetics and practicing austerity. Presumably he learned a lot during these times, though emphasis seem to be on things he learned that weren't fruitful.
But things that aren't fruitful aren't the whole story of this period. For example, he found some teachings of his previous teachers useful and kept teaching and exercising them even after Nirvana (e.g. meditation on nothingness/suññata) -- though these practices, by themselves, were remarked as insufficient for realizing Nirvana.
Another example of useful practice is the time when he decided to divide his thoughts in two classes:
“Bhikkhus, before my enlightenment, while I was still only an unenlightened Bodhisatta, it occurred to me: ‘Suppose that I divide my thoughts into two classes. Then I set on one side thoughts of sensual desire, thoughts of ill will, and thoughts of cruelty, and I set on the other side thoughts of renunciation, thoughts of non-ill will, and thoughts of non-cruelty.
[...] “As I abided thus, diligent, ardent, and resolute, a thought of cruelty arose in me. I understood thus: ‘This thought of cruelty has arisen in me. This leads to my own affliction, to others’ affliction, and to the affliction of both; it obstructs wisdom, causes difficulties, and leads away from Nibbāna.’ When I considered: ‘This leads to my own affliction,’ it subsided in me; when I considered: ‘This leads to others’ affliction,’ it subsided in me; when I considered: ‘This leads to the affliction of both,’ it subsided in me; when I considered: ‘This obstructs wisdom, causes difficulties, and leads away from Nibbāna,’ it subsided in me. Whenever a thought of cruelty arose in me, I abandoned it, removed it, did away with it.
-- MN 19
An insight (no pun intended) on the kind of person Gotama was before Nirvana can be seen from the following passage, where he was training severe ascetism:
“I would make my bed in a charnel ground with the bones of the dead for a pillow. And cowherd boys came up and spat on me, urinated on me, threw dirt at me, and poked sticks into my ears. Yet I do not recall that I ever aroused an evil mind of hate against them. Such was my abiding in equanimity.
-- MN 12
That is, even in extreme circumstances he already had some extraordinary domain over his mind or such a virtuous persona that unwholesome states of mind would not invade him in these circumstances. Also, the whole narration of the ascetism period shows how deeply dedicated he was.
That much can be read in the suttas from the period pre-Nirvana.
Many buddhists believe that this is due to his many previous lives acquiring merit and becoming such a person that would be able to, by himself, become the Buddha -- the Jataka stories illustrate that.
A complementary point of view could be to take the Dharma that he started teaching. As far as we know, he didn't taught vipassana for some 40 years, he taught the eightfold path. He also implied that the eightfold path is intrinsically the path Nirvana -- seeing one is seeing the other. In other words, this same path was discovered by all Buddhas.