Is my not speaking to the young man whom God has destined to be my fiancé because I made a vow to my Lord, or because speaking to him is forbidden?

Love relationships and conversations between young men and women online are not sanctioned by Sharia and are a gateway to fitna (temptation/strife) and evil. You both must repent to Allah and sever this relationship. If the young man wishes to marry, he should propose to your family. Regarding your vow to Allah to stop conversing with him: if it was a verbal oath, then you must perform an expiation for breaking an oath (feeding ten needy persons or clothing them, or fasting for three days), in addition to repenting. If it was a promise not in the form of an oath, then you do not owe an expiation for breaking an oath, but you have violated a promise to Allah and fallen into what is forbidden, so you must repent and seek forgiveness.