2,749. The Differences Between a Vow and an Oath
Hilchos Nedarim 3:1
There are four differences between a vow and an oath of expression: (1) a new oath will not take effect on top of a previous oath, while a vow can take effect on top of a previous vow; (2) if a person tries to expand an existing oath, he isn’t liable, but one is liable for this in a vow; (3) an oath of expression can only take effect vis-à-vis things about which one has a choice, while vows can also take effect vis-à-vis mitzvos; d) an oath of expression can take effect regarding something that has substance and something that doesn’t have substance, while a vow can only take effect regarding something that has substance.
Hilchos Nedarim 3:2
“A vow can take effect on top of a previous vow” means that if someone says, “I will be obligated in an offering if I eat this loaf” and then he says, “I will be obligated in an offering if I eat it,” he must bring an offering for each vow. The same is true in all comparable cases.