I grew up spending summers on the lake and my Granny always made this to have on hand and would slather this onto crackers for us when we came in from a swim. Easy to spread on bread for sandwiches, too.
Brin the sugar and water to a boil. Let cool. Add vanilla, peanut butter and marshmallow fluff. Mix well. Mixture will be thin so let it sit overnight. Granny never kept this in the fridge, but neither did she keep pickles, jams, or ketchup/mustard in there either, and none of us has died from it yet, but I suppose I am supposed to say "keep refrigerated" just for liablility purposes. So, yeah, do that (if ya want).