n. — proprietary, Saskatchewan, Food & Drink
chocolate milk (see Image 1).
Type: 1. Origin — The term was originally an abbreviation for "vitamin concentrate" (see the 2008 quotation), and later became generalized in Saskatchewan to mean all chocolate milk. Vi-Co was trademarked in 1933 and again in 1957 by Chocolate Products Limited, a company based in Ontario (see Canadian Trade-marks Database references). The Saskatchewan connection is profound (see, e.g., the 2007 quotation) and is likely a preservation and generalization of this early 20th-century trade name (much like "Kleenex" for paper tissue, for instance). The term is most prevalent in Canada (see Chart 1), of which almost all uses come from Saskatchewan, although our regional internet resources did not allow for the creation of a chart.
See also COD-2, s.v. "Vi-Co", which is marked "Cdn (Sask.) proprietary".
Images:

Image 1: A litre of Vi-Co. Source: http://laiteriesduquebec.com/divers/divers.htm. Photo: D. Morin 
Chart 1: Internet Domain Search, 16 Oct. 2012