Is fasting three days sufficient as an expiation for a vow to apologize to a colleague after I recover from what afflicted me, or is it obligatory to apologize to him?

You did well if you intended to close the door to evil by speaking to him in an unfavorable way to stop him from contacting you. And your vow to apologize to him is not a vow of obedience, but rather a vow concerning a permissible act, and it is not binding. If your apology leads to evil, or results in a submissive tone of voice, or a private meeting, then it is not permissible to fulfill it, and no expiation is binding upon you.