Thus I continued to travel. Sometimes by land, sometimes by water...till, at the end of six or seven months after I had been kidnapped, I arrived at the sea coast... The first object which saluted my eyes when I arrived on the coast was the sea, and a slave ship, which was then riding at anchor, and waiting for its cargo. These filled me with astonishment, which was soon converted into terror when I was carried on board. I was immediately handled and tossed up to see if I were sound... and I was now persuaded that I had gotten into a world of bad spirits.... When I...saw...a multitude of black people...chained together, every one of their countenances expressing dejection and sorrow, I no longer doubted my fate... (Equiano, pp. 69-70).
He left Africa probably via the slave port of Bonny, circa 1754. He was named Michael on board the slave ship. Vassa documents in detail the horrors of the Middle Passage: the brutal conditions of the slaves on board the slave ship, the trepidation which accompanied them during the crossing and the  
perception of himself and his unfortunate mates that the white men on board were cannibals who meant to eat them:  
I was soon put down under the decks, and there I received such a salutation in my nostrils as I had never experienced in my life: so that, with the loathsomeness of the stench, and crying together, I became so sick and low that I was not able to eat, nor had I the least desire to taste anything. I now wished for the last friend, death, to relieve me... (Equiano, pp. 69-70).  
As a political document, such a detailed, visceral description of life on board the slave ship in Vassa's autobiography was very useful in eliciting sympathy, and Vassa was well aware of this.