In Vassa's childhood, men from certain south-eastern Igbo communities painted their bodies red with camwood. Furthermore, Vassa also documents the names of the masters he served and the dates he served them, details that would have been easily verifiable in his days, I imagine. Moreover, these speculations were also rife in Vassa's lifetime, meant as he saw it to discredit his work and prevent the sale of his Narrative (Equiano, p. 255). Lovejoy notes that:  
The attack on Vassa's credibility as a witness to slavery, the form it took and the timing on 25 April 1792, were a commentary on the abolition movement. The Danish edict had been enacted on 16 March, and while it did not end the Danish involvement in the slave trade for another decade, it inspired Wilberforce's renewed motion to abolish the slave trade (Lovejoy, p. 174).  
In a later edition of his autobiography, in 1792, Vassa calls as witnesses, 'respectable persons of character who knew me when I first arrived in England, and could speak no language but that of Africa' (Equiano, p. 251). Those persons were presumably still alive and could attest to the truth of his story. The fact that these speculations persist today, and the fact that they are rigorously defended are testament to Vassa's enduring legacy as a historical figure and as a writer.  
Despite the speculations, Vassa's story and his written work remain an important window into the social, cultural and economic life of the Igbo society before European colonization, and his accounts of the horrors of the slave trade made a substantial contribution towards its abolition. The Interesting Narrative of the Life of Olaudah Equiano, or Gustavus Vassa, the African, Written by Himself remains as important today as it was when first published.