forbade vowing and said: "Indeed, it does not bring any good; rather it is only a means to extract something from the miser

It is not prescribed in the Shariah to make a vow, but if a person makes one, he is obligated to fulfill it, unless it involves a sin.
The cause for the forbiddance is that "it does not bring good", as it cannot ward off anything decreed by Allah, and lest a vow maker may think that the fulfillment of his need was the result of the vow. Allah Almighty is Free of need for that.
Al-Qurtubi said: This forbiddance lies where one may say, for example: If Allah cures my sick person, I shall give such and such in charity. The reason behind dislike is that when he made the mentioned act of piety dependent upon the mentioned purpose, it appeared that he did not have the intention to draw close to Allah Almighty for what He did, but he took the path of compensation in this regard. This is made clear by the fact that if He did not cure his sick person, he would not give the charity he made conditional upon this cure. Such is the attitude of a miser, who does not give anything of his money without immediate compensation that is mostly higher than the given amount.