After eight winters with the same down parka, here is the short version of how to store winter coats in summer: clean it, dry it completely, keep it loose, and put it somewhere dark and dry. Everything else is decoration. Down and wool mostly die of moisture, compression, and pests, and all three problems begin …
Short answer first: vacuum sealing down jackets isn’t inherently bad, but leaving one sealed for six straight months is — I have a flattened 700-fill jacket in my closet to prove it. Compression is mostly recoverable; duration, trapped moisture, and how you re-loft the jacket afterward decide the outcome. Fix rips first, since down jacket …