When freezing any kind of food it will lose some of its vitamins, this is why it's not recommended to leave F/T frozen for too long. Just because something is frozen doesn't mean it absolutely ceases to break down or change, the freezing just slows the process of change, so it just takes a lot longer. Freezer-burn, for example, is what happens when foods are frozen for too long.

Regardless, the amount of vitamin loss shouldn't be too dramatic or detrimental in the long run, just try not to buy in mass and slowly use your F/T. Buy supplies for a smaller chunk of time so that the food isn't frozen too long. :]

(Though I feed pre-killed anyways. Frozen food just seems inherently odd to me, lol..)