We find scaling behavior of the linear ac impedance consistent with a transition into a vortex glass state in measurements on cuprate YBaCuO superconducting thin films in large magnetic fields, H≳Hcl. The films were measured in the frequency range from 100 kHz to 500 MHz. At the vortex glass transition temperature, Tg, and in the ac linear regime we find scaling behavior of the impedance amplitude, ∥Z∥∝ω 0.83, and a frequency-independent phase, φ=74°. The measured frequency dependence, ∥Z∥∝ω(2-d+z)/z, and phase φ=(π/2)(2-d+z)/z is consistent within experimental error of the value of the critical exponent z obtained from dc current-voltage measurements on the same sample. At higher temperatures and still in the linear regime, we find scaling behavior of a temperature-dependent crossover frequency to the critical regime, Ω(T-Tg), and at Tg, scaling behavior of a current density which sets the size of the linear regime, J0(ω, T=Tg). Within experimental error, all of these results are in excellent agreement with predictions for the ac impedance of the vortex glass phase transition in large magnetic fields.