When you deal damage to a Dynamax Pokemon in a raid, you’ll notice that sometimes, even when the shield is up, the dynamaxed Pokemon will still take some damage. The shields are there to reduce damage instead of completely walling the Pokémon from damage. It could have been very likely that the Vibrava had terrible special defense IVs, was on low health, or that your Frosmoth was at a much higher level. These things happen, even to me. Additionally, Vibrava is 4 times weak to ice, meaning that your damage output would have been extremely high nonetheless,