I like to keep a supply of butter in the freezer. Look for it on sale and pack it away. Usually have 15-20 lbs. on hand and try to rotate it. Sometimes a lb. or two gets lost in the back or bottom of the freezer and absorbs freezer smell.

The question is, will vacuum seal bags keep the smell out or would it be better to use mylar bags?
Thanks.

I would guess that mylar would preform better, but either would be better than none.  Even plastic food wrap or a ziploc style bag would be better than just the thin wax covered cardboard box and piece of wax paper it comes wrapped in.

For long term storage, try canning butter.

The Ultimate Guide To Canning Butter At Home

Sort of shows threadjack bc OP's question is about bags for sealing it up, but don't overlook canning clarified butter or ghee. I do both and it lasts well. I also vac seal and store frozen butter.

OP, mylar is probably best but any sealed bag is better than unsealed. Unsealed butter will lose moisture and pick up odors.

I last bought butter pre-covid when I found a combination coupon & sale price that got name brand butter down to $1.30/lb. I stored over 50 lbs of it in vac bags in the freezer.