I suspect it's best to freeze only unflavored nicquid for long-term storage. The flavors might well change (and not for the better) after some years below zero.
I freeze 100 mg./ml. unflavored rocket fuel, then bring up to room temperature and DIY (dilute with PG and/or VG, then add flavor) small batches.
My drop-in freezer fluctuates between -5 and -15 degrees Farenheit. I'll leave frozen steaks and porkchops in there for months without worry. But once I lost a package of hot dogs in there for about four or five years (excellent, German-style, skin-on dogs in cryo-pack purchased on one of our forays beyond the Cheddar Curtain). I thawed them, then lovingly simmered them. They hadn't gone bad, in the nasty bacterial sense, but they definitely tasted "off."