Did Buddha Return to Extreme Asceticism?
By this point, the Future Buddha had tried extreme mortification, realized that wasn't working, returned to eating to sustain his life, went under the Bodhi Tree, then makes the vow to not leave again until he finds the answer, EVEN til his body is dried and shriveled.


The ascetic practices he did were the methods he thought would lead to some spiritual attainment. He thought that not eating and making his body weak could have some benefit. 

Going to the Bodhi tree and not leaving is not a method. He didn't think that the act of not leaving the shades of a certain tree was a particularly interesting practice on its own merit.

The vow is interpreted not as asceticism, but as resolution or commitment. Whether he really meant that he would die under the tree we can't tell, but by the time he took food, it's clear that he was giving up the idea of trying to inflict pain in his body as a means to spiritual progress. Finally, it's clear that the practices he undertook under the tree were not ascetic in nature.