A vow in intention only does not have to be fulfilled
Question

                Eight years ago I vowed to fast an entire month but it was only an intention. After a short while I changed my mind because I thought it would be too difficult to fulfil it, and I vowed to fast three days every month, and that was a verbal vow. I have continued to fulfil the vow that I spoke out loud for the past eight years. Do I have to fulfil the first vow (which was intended only) or the second one which was verbal? If I have to fulfil the second vow, is the first vow waived?.
Praise be to Allah.
    The first vow which was not verbal does not have to be 
    fulfilled, and the second vow which was verbal, and in which you promised 
    Allaah that you would fast such and such, or read such and such (of Qur’aan), or give such and such in charity, this action is an act of worship and this vow must be fulfilled. But if it was only an intention and was not verbal, then it does not have to be fulfilled. End quote.