Q. If a person pledged on Simchas Torah to learn a certain masechta by a certain deadline, and within the masechta there are some parts that he does not understand (difficult Gemaras), how should he comply with his vow?
Should he be matir neder?

A. Horav Shlomo Miller’s opinion is that he should try his best and daven to Hashem to help him understand the difficult passages. He does not have to be matir neder.
When making any pledge one should always say “beli neder,” so he wont incur in the prohibitions involving nedarim or oaths.
See also question 1435 regarding the different opinions on what is considered the bare minimum of having learned something, for siyum purposes.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a