A Study to Compare Treatment With the Drug Selumetinib Alone Versus Selumetinib and Vinblastine in Patients With Recurrent or Progressive Low-Grade Glioma

Inclusion Criteria: * Feasibility phase: patients must be \>= 2 years and =\< 21 years of age at the time of enrollment * Efficacy phase: patients must be \>= 2 years and =\< 25 years of age at the time of enrollment * All patients \> 21 years of age at the time of enrollment must have had initial diagnosis of low-grade glioma by 21 years of age * Patients must have a body surface area (BSA) of \>= 0.5 m\^2 at enrollment * Patients must have eligibility confirmed by rapid central pathology and central molecular screening reviews performed on APEC14B1 * Non-neurofibromatosis type 1 (non-NF1), non-tuberous sclerosis complex (non-TSC) low-grade glioma (LGG) without a BRAFV600E or IDH1 mutation * Patients must have progressive or recurrent LGG. Note: Biopsy may be at either initial diagnosis or recurrence * Patients must have measurable disease, defined as having a two-dimensional measurable tumor volume of \>= 1 cm\^2 * Tumor size will be measured to include both solid and cystic components of the tumor (whether or not tumor is enhancing) + fluid attenuated inversion recovery (FLAIR) signal * Eligible histologies will include all tumors considered low-grade glioma or low-grade astrocytoma (World Health Organization \[WHO\] grade 1 and II) by the WHO Classification of Tumors of the Central Nervous System - 4th Edition Revised, with the exception of subependymal giant cell astrocytoma * Patients with metastatic disease or multiple independent primary LGGs are e…