There will always be some debate surrounding the terminology, but basically what we're talking about is:
Vintage -- A handbag made in the style of an era during that era. A bag is vintage if it is from a prior time in fashion, and especially if it is recognizably from that time (meaning it embodies certain styles popular in that period). Your great-grandmother's beaded flapper bag from the 1920s is a vintage piece. A vintage bag is an antique (and could be worth some cash).
Retro -- A handbag made in the style of an era after that era has passed. A bag is retro if it recalls a prior time in fashion. The '70s-style printed hobo bag on the rack in Macys in 2010 is retro, not vintage, and it's only worth the MSRP on the price tag.
You could, in theory, have a vintage retro bag -- say, a Victorian-style beaded pouch crafted in the 1960s.
The bag that graced Gucci's runway was retro. It's new, and it recalls one of the great classics.
And speaking of classics…