My wife owned some gold and said: "If I sell it, I will give its price as charity (continuous charity)." Is it considered a vow? Is it permissible for her to buy shares in cooperative societies and then give charity later on from the shares' profits in an amount equal to the value of the sold gold?

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

Your wife's statement is not considered a vow. Rather, it is just a promise to give the value of gold as charity. She is blameless for not fulfilling the promise although it is better to fulfill it.

It is permissible to buy shares in cooperative societies and then later give the charity out of the profits in an amount equal to the value of the sold gold provided that these societies engage in permissible activities. It is conditioned that they do not deal in interest-based loans.

It is also better to hasten to give what she intended to give in compliance to the command of hastening to do what is good.  Allaah Almighty says (what means): {And hasten to forgiveness from your Lord and a garden as wide as the heavens and earth, prepared for the righteous, who spend [in the cause of Allaah] during ease and hardship.} [Quran 3: 133-134]

Allaah Knows best.