Vintage is an item quality used most commonly to designate previously Unique items obtained prior to the Mann-Conomy Update, whence it was introduced. The intent was to increase their appeal in trading as well as offer a refresh to the economy.
In the March 15, 2011 Patch, several expensive crafting recipes were modified to be cheaper. To recognize items that had been crafted while the recipes were expensive, Valve upgraded them to Vintage quality. Vintage items are functionally and cosmetically identical to their Unique counterparts.
In some cases, items that have been re-granted to users by Steam Support (after being either accidentally deleted or stolen) were assigned Vintage quality, although Support no longer grants back lost items in nearly all cases as of late 2016, there are some notable exceptions such as the spoofed item scam from Steam game developers in late July 2018.  Other exceptions have been items granted for participation in certain events.
The hexadecimal color code for Vintage items is.
List of Vintage items