These two texts are very interesting in the way that they examine both slaves and women as being treated as less than human in various ways, even though, arguably, it is possible to say that the way women are treated in Wollstonecraft's essay is not as severe as the experience of being a slave as recorded by Gustavas Vassa. Mary Wollstonecraft for example refers to the ways in which women are infantalised and treated as if they were fragile dolls by men, unsuited for the real world. Note the following quote:

My own sex, I hope, will excuse me, if I treat them like rational creatures, instead of flattering their fascinating graces, and viewing them as if they were in a state of perpetual childhood, unable to stand alone.

Wollstonecraft's irony is evident in the way that she asks to be "excused" for treating women as humans rather than dependent people who are unable to stand on their own two feet and who need constantly to have their beauty reinforced. Wollstonecraft argues throughout her essay that women are treated as the lesser and inferior gender, and that this constitutes a kind of abuse through the way that women are not allowed to become independent and are not treated as such. 

Oloudah Equiano is famous for his account of the Middle Passage, the trip that so many slaves made from Africa across to the West Indies where they were sold. The following quotation is taken from his account of this journey and its record of the way slaves were treated is shocking in the extreme. It begins by talking of the conditions in the hold, where the slaves were kept in highly unsanitary conditions:

This produced copious perspirations, so that the air soon became unfit for respiration, from a variety of loathsome smells, and brought on a sickness among the slaves, of which many died, thus falling victims to the improvident avarice, as I may call it, of their purchasers. This wretched situation was again aggravated by the galling of the chains, now become insupportable; and the filth of the necessary tubs, into which the children often fell, and were almost suffocated. The shrieks of the women, and the groans of the dying, rendered the whole a scene of horror almost inconceivable.

Note the reference to the "avarice... of their purchasers," suggesting that those buying slaves were so desperate to have as many as possible that the slave ships were crammed way beyond their capacity, creating terrible conditions in which many slaves died. 

In terms of comparing the two accounts therefore, it is clear that in one women are treated as less than human, whereas in the other, slaves are treated as not even human at all, and are presented more as animals than anything else. Both are similar in the way that women and slaves are not treated as being equal to either men or whites, respectively, and are treated as if they were a substandard and inferior creature.