What is the ruling on someone who vowed to fast three days, and fasted one day then fell ill on the second, so he did not complete his fast fearing pain, and then the period of the vow expired?

You must seek forgiveness for your shortcoming in fulfilling the vow, and make up for the two days of fasting. You also owe an expiation for an oath (kaffarat yameen) due to the failure to specify (which is fasting the days before the end of the year). This is the view of the Hanbalis. Some scholars (Malikis and Shafi'is) are of the opinion that you only need to make up for the two days without expiation. The first opinion is more cautious.