Is it permissible to feed the poor instead of fasting a full month vowed to Allah, while being capable of fasting?

A vow must be fulfilled in the manner in which its owner made it. It is not permissible to change it or substitute it with something else unless one is completely unable to fulfill it and there is no hope of this inability ceasing. In this case, one must offer an expiation similar to that for breaking an oath. Therefore, it is not permissible to feed the poor instead of fasting for a full month, as long as the questioner is able and has not experienced an inability that is unlikely to cease.