"And now the Saxon chief prepared an entertainment, to which he invited the king, his officers, and Ceretic, his interpreter, having previously enjoined his daughter to serve them so profusely with wine and ale, that they might soon become intoxicated."
Here we go. Interesting that Nennius calls Hengist "the Saxon chief" and Vortigern "king". They also needed an "interpreter", thus indicating that their languages were by this time separate and distinct. But the plan was simple. Get everyone drunk and let the hormones fly!
"This plan succeeded, and Vortigern, at the instigation of the devil, and enamoured with the the beauty of the damsel, demanded her, through the medium of his interpreter, of the father, promising to give for her whatever he should ask. Then Hengist, who had already consulted with the elders who attended him of the Oghgul race, demanded for his daughter the province, called in English Centland, in British, Ceint, (Kent). This cession was made without the knowledge of the king, Guoyrancgonus who then reigned in Kent, and who experienced no inconsiderable share of grief, from seeing his kingdom thus clandestinely, fraudulently, and imprudently resigned to foreigners. Thus the maid was delivered up to the king, who slept with her, and loved her exceedingly."
Quotes taken from: Medieval Sourcebook: Nennius: Historia Brittonum, 8th century