Dreamless state and Samadhi share the condition of absence of any Vrittis. Can they be the same thing then? If not where is the difference?
No they are completely different. Many people have dreamless sleep at any given time. They do not show any change in their personality after sushupti. It is completely different after the experience of Samadhi.
We have taken a cursory view of the different steps in Râja-Yoga, except the finer ones, the training in concentration, which is the goal to which Raja-Yoga will lead us. We see, as human beings, that all our knowledge which is called rational is referred to consciousness. My consciousness of this table, and of your presence, makes me know that the table and you are here. At the same time, there is a very great part of my existence of which I am not conscious. All the different organs inside the body, the different parts of the brain — nobody is conscious of these.
When I eat food, I do it consciously; when I assimilate it, I do it unconsciously. When the food is manufactured into blood, it is done unconsciously. When out of the blood all the different parts of my body are strengthened, it is done unconsciously. And yet it is I who am doing all this; there cannot be twenty people in this one body. How do I know that I do it, and nobody else? It may be urged that my business is only in eating and assimilating the food, and that strengthening the body by the food is done for me by somebody else. That cannot be, because it can be demonstrated that almost every action of which we are now unconscious can be brought up to the plane of consciousness. The heart is beating apparently without our control. None of us here can control the heart; it goes on its own way. But by practice men can bring even the heart under control, until it will just beat at will, slowly, or quickly, or almost stop. Nearly every part of the body can be brought under control. What does this show? That the functions which are beneath consciousness are also performed by us, only we are doing it unconsciously. We have, then, two planes in which the human mind works. First is the conscious plane, in which all work is always accompanied with the feeling of egoism. Next comes the unconscious plane, where all work is unaccompanied by the feeling of egoism. That part of mind-work which is unaccompanied with the feeling of egoism is unconscious work, and that part which is accompanied with the feeling of egoism is conscious work. In the lower animals this unconscious work is called instinct. In higher animals, and in the highest of all animals, man, what is called conscious work prevails.
But it does not end here. There is a still higher plane upon which the mind can work. It can go beyond consciousness. Just as unconscious work is beneath consciousness, so there is another work which is above consciousness, and which also is not accompanied with the feeling of egoism. The feeling of egoism is only on the middle plane. When the mind is above or below that line, there is no feeling of "I", and yet the mind works. When the mind goes beyond this line of self-consciousness, it is called Samâdhi or superconsciousness. How, for instance, do we know that a man in Samadhi has not gone below consciousness, has not degenerated instead of going higher? In both cases the works are unaccompanied with egoism. The answer is, by the effects, by the results of the work, we know that which is below, and that which is above. When a man goes into deep sleep, he enters a plane beneath consciousness. He works the body all the time, he breathes, he moves the body, perhaps, in his sleep, without any accompanying feeling of ego; he is unconscious, and when he returns from his sleep, he is the same man who went into it. The sum total of the knowledge which he had before he went into the sleep remains the same; it does not increase at all. No enlightenment comes. But when a man goes into Samadhi, if he goes into it a fool, he comes out a sage.
What makes the difference? From one state a man comes out the very same man that he went in, and from another state the man comes out enlightened, a sage, a prophet, a saint, his whole character changed, his life changed, illumined. These are the two effects. Now the effects being different, the causes must be different. As this illumination with which a man comes back from Samadhi is much higher than can be got from unconsciousness, or much higher than can be got by reasoning in a conscious state, it must, therefore, be superconsciousness, and Samadhi is called the superconscious state.
The Complete Works of Swami Vivekananda, Volume 1, Raja Yoga, Dhyana and Samadhi
As I mentioned in my comment, I am not an expert in samAdhi, I don't think the main upanishads talk about samAdhi, which would be surprising if samAdhi were really fundamental to brahmajnAna.
I can say a few words about sushupti though. Now there are two kinds of interpretations of sushupti, even within the advaita-vedAnta school. There are direct statements in some of the upanishads that suggest that sushupti is the state of highest bliss, equating it to the supreme non-dual state. Somebody soon found out that this kind of teaching would create a problem since liberation is permanent but sushupti is temporary. After all, we wake up from sushupti and find the world exactly as it is, before going to sleep. This cannot be explained if sushupti = liberation and if nothing else exists in the state of sushupti. Probably that is the reason why the mANDUkya postulates a fourth "state" of turIya (= liberation) that is the witness of other 3 states. But some other upanishads treat the sushupti as samprasAda or highest serenity.
Now advaita vedAnta holds that avidyA is the reason for duality. The question is whether avidyA exists in the state of sushupti or not. Orthodox advaitins argue that if avidyA did not exist in some form in sushupti, then waking up from that state is impossible. So they hold that avidyA exists in the seed form in that state. Seed form because the effects of avidyA are suppressed in that state. However, some upanishads say that sushupti is a state of non-duality.
Both kinds of teachings are present in Shankara bhAshyas. Swami Ishwarananda, in his book God realization through reason gives a list of quotes from the upanishads and Shankara bhAshyas that treat sushupti as the highest state.
Sushupti is like a hidden treasure of gold beneath one's feet. One passes over it everyday, not realizing it's worth. (One goes to deep sleep everyday without realizing it is the supreme state).
Sushupti is the samprasAda, the highest serenity and highest bliss.
Sushupti is the supreme non-dual state. There is no second thing in that state.
avidyA, desires, objects, work are all absent in the state of sushupti. (Orthodox advaitins say that avidyA is present in seed state).
sushupti state is "proof" of liberation because it shows that avidyA is accidental and not natural to the Self (for those who hold that avidyA is absent in deep sleep).. A father is not a father, a mother is not a mother, the veda are not vedas, a chandAla is not a chandAla etc in that state. The state is free from all impurities.
One does not know anything in sushupti because one becomes unified with supreme brahman. There is no second thing to be known, but the ability of the knower to know persists.
Sushupti is not due to a mental modification. It is the supreme state of brahman.
Sushupti is our natural state of the Atman. Hence one becomes refreshed after a good night's sleep because one goes into his or her natural state during sushupti.