This is disliked to do but if one made such a vow, he should fulfill his promises. We are limited and don’t know what is the best for us. But Allah knows.  Watch this please:
http://www.youtube.com/watch?v=pHwq9_xTrbE
http://www.youtube.com/watch?v=XniybghRzbw

If some one wants dua to be answered -let’s say to get married ,get a good job or have kids and the like -can one make niyyah to Allah that i will fast a a couple of days or make some amount of nawafil prayers or give sadaqah and the like if my certain dua gets answered.