Jacob then made a vow, saying, “If God remains with me, protecting me on this journey that I am making, and giving me bread to eat and clothing to wear, Genesis.28.20  

 Ohr HaChaim The proper explanation of what Jacob said is that he was prepared to conduct himself in such a way that G'd would consent to associate His name with him even during his lifetime.   

R' Bachya Yaakov wanted to teach his future offspring that the time to make vows is when one finds oneself in difficulties one does not know how to extricate himself from. Although, as a general rule, the Torah frowns on the making of vows, when a person vows due to finding himself in distress this is perfectly permissible.