1) It is correct that one should abstain even from making vows that one will contribute to charity. Instead, if one has sufficient money, one should contribute immediately, and if at this moment he has not enough, he should wait until such time as he has enough and give without a vow. If they ask him in public to give a certain sum of money to charity, or he is promising money at the time when one mentions the departed souls, he should insist that it is all being done without a vow. (Kitzur Shulchan Aruch 67).
2) In times of trouble one may make a vow. (ibid).