they dont have the same drop rate, and ill find you some official info:
later on they stopped adding that drop rate info part but the rates didnt change
if you compare gem chests and vip chests costs you will see that vip chests have higher chance then gem chests, even considering the costs
you can find all over the forum comments about “how vip chests dont drop mythics as often as they should” and that “gem chests drop them more likely” but i would rather bet it is just rng bias on ppls expectations