Probably, the story of the vow was an exaggeration in the Jataka, and seems to hint of extreme asceticism.
Perhaps, the truth behind the actual story might be the Buddha's resolve and effort i.e. Right Resolve (aka Right Intention) and Right Effort. It was more of a resolution and a decision, rather than a vow or a promise.
As you can see in the sutta quote below, too little energy results in laziness. Too much energy results in restlessness.
However, the right amount of energy, along with the rest of the Noble Eightfold Path, will bring the right results.
So, basically, the Buddha resolved to apply the right amount of energy into his effort.
“What do you think, Soṇa? When you were still a layman, weren’t you a
good harp player?”
“When your harp’s strings were tuned too tight, was it resonant and
“When your harp’s strings were tuned too slack, was it resonant and
“But when your harp’s strings were tuned neither too tight nor too
slack, but fixed at an even tension, was it resonant and playable?”
“In the same way, Soṇa, when energy is too forceful it leads to
restlessness. When energy is too slack it leads to laziness. So, Soṇa,
you should apply yourself to energy and serenity, find a balance of
the faculties, and learn the pattern of this situation.”
“Yes, sir,” Soṇa replied.