It is said that Lord Krishna was a human being with qualities of God, thus Lord Krishna has a special place in sanatana dharma. I would like to understand, if Veday Vyasa has written Mahabharata, whether he has seen Lord Krishna and therefore he was able to write so much about Lord Krishna in Mahabharata and later in Bhagavatham. I also have read that Veda Vyasa is also known as 'Krishna Dwaipayan'. Does it mean that Veda Vyasa himself is krishna?
Yes. Vyasa saw and met Sri Krishna many times but this wasn't how Vyasa wrote Mahabharata. One such meeting happened during Brahmastra episode of Arjuna and Aswathama. Indeed, Kauravas are biological descendants of Vyasa.
Vyasa with his Trikaladarshi powers analyzed Vedas and taught Puranas to his disciples. Here is what Chapter one of Adi Parva of Mahabharata says:
The son of Satyavati having, by penance and meditation, analysed the eternal Veda, afterwards composed this holy history, when that learned Brahmarshi of strict vows, the noble Dwaipayana Vyasa, offspring of Parasara, had finished this greatest of narrations, he began to consider how he might teach it to his disciples.
And it was thus that Vyasa was born of Satyavati through Parasara. And because he was born in an island, he was called Dwaipayana (Dwaipa or islandborn). And the learned Dwaipayana, beholding that virtue is destined to become lame by one leg each yuga (she having four legs in all) and that the period of life and the strength of men followed the yugas, and moved by the desire of obtaining the favour of Brahman and the Brahmanas, arranged the Vedas. And for this he came to be called Vyasa (the arranger or compiler).
Since Krishna Dwaipayana Vyasa is Vishnu and Krishna is also Vishnu, you can consider Vyasa as Krishna though they differ in some qualities or attributes.