A "Vintage Ski Sweater", should be considered any ski sweater of at least five various types depending on how people define the word vintage:
vintage (comparative more vintage, superlative most vintage)
1 (attributively) Of or relating to a vintage, or to wine identified by a specific vintage.
2 (attributively) Having an enduring appeal; high-quality
3 (attributively) Classic (such as video or computer games from the 1980s and early 1990s, or old magazines, etc.).
4 (attributively) Of a motor car, built between the years 1919 and (usually) 1930 (or sometimes 1919 to 1925 in the USA).
5 Of a watch, produced between the years 1870 and 1980.
Personally when I hear the term used in this sense with clothing I generally take it to mean that they are of a classic design and are generally more pricy than other styles.