I get an impression that certain parts of vedas proclaim montheism, others polytheism, some trans-theism and yet others atheism. Is it not internal contradiction?
Your question is a good one and I am happy to attempt to share my understanding.
The reason there may seem to be contradictions is simply because there is a difference in perspective.From the standpoint of the limited, there is a limitless, from the standpoint of a someone who is limited in time and space, there is an eternal, infinite, presence.
There are no two opinions about this.
Vedanta's beauty is in going beyond this subject-object construct, in breaking through this matrix and saying that the limited, the jiva, the bound, only has a notional existence. There is no "real" thing called a jiva who is ever at any point in time "separate" from the whole "Ishwara" and that these divisions are only mithya.
At all times there is only One, vastu or existent entity and it is Truth, it is Consciousness and It Alone Is, One, without a second. You may choose to call it Brahman or God as well.
In your dream, there is a entire universe you create. And yet you are not intricately involved with anything in the dream. Time passes, there may be multiple catastrophes, and upheavals in that dream creation, yet, you the sleeper remain untouched.
From the standpoint of the multitude of entities in the dream, they all have a sense of separation - both from each other as well as from you the dreamer.
But the reality is that you the dreamer alone are the dream. No aspect of the dream is ever separate from you.
So from the perspective of the dreamworld you are the dream creator, sustainer and destroyer.
But there is an aspect about you that is unrelated to the dream.
Even after the dream is over you still exist. The dream now exists only in potential form.
The same it is with Brahman or God. From the standpoint of this manifest universe, He is both the efficient and material cause. From a transcedental standpoint, He alone Is. Thus "transtheism", or if you transcend the jiva's existence as being mithya, "atheism" (because there is now no other thing you can speak of)
From the wordly standpoint, there are any number of deities presiding over varying aspects of creation. And that is because the Divine is immanent in and through every aspect of manifest creation. Hence alone, in our culture, each and everything is sacrosanct. So you look at the Sun - and ask - is this God? The answer is a resounding yes. It is God. After all what is not God in creation. So if you invoke the Lord in the Sun you can give a certain name and form to him - and call it Lord Surya. And you can symbolize the seven rays as a chariot with seven horses. And if you invoke the very same divinity in the Earth, you can call it Bhoomi Devi. And so on and so forth.
So yes there is polytheism. But what is worshipful or who is worshipped are at no point multiple entities - it is the very same entity that is invoked at different times and by different people based on their temperaments. Thus monotheism.
To give an example, if it is dark I switch on the light, if it is hot i switch on the fan, - at both times the only thing i am really invoking is electricity. The names light and fan are really only different forms in which electricity can bless me depending on which form i choose to invoke it in.
So all these "-isms" whether polytheism or monotheism or atheism or transtheisms are all valid concepts which all have their own place, as long as there is an understanding of the overall construct in which they are placed.