You have just committed one of the most heinous sins in Islam which made Iman exit your heart and you are concerned by the expiation of your oath?
Are you serious?
You have a real problem if you are unable to measure the gravity of your sins!
Expiating an oath is feeding ten poor people and it has nothing with the sin of fornication! This woman is a prostitute without a doubt!

if a person promises to never go to a prosititute , but then goes to a massage place and has illegal intercourse with the women , does he have to give the kaffarah for breaking the promise? or is it okay not to give it as the massage women is not classed as a prostitute.