Round hole or “f” hole? Kind of depends on body size and if an “f” hole, whether it has a cutaway or not. Ebay “completed auctions” are always a good source of mkt info. In original condition and not thrashed, it could be worth $800-$1200.
Here’s an older one that should sell in that range:
http://cgi.ebay.com/martin-vega-1939-c56-guitar-great-for-collecters_W0QQitemZ130152827262QQihZ003QQcategoryZ2385QQrdZ1QQssPageNameZWD9VQQcmdZViewItem
Almost looks like a Gibson body, perhaps from an L-48 or L-50.
Vega, after the late 50’s, was both a manufacturer and a “relabeler” of guitars made by other cos. So, a Vega from that era could be a relabeled Harmony or Kay (fairly cheapo) or a genuine Vega-made which would be worth a lot more. They also bought bodies from “X” and put their necks on, that type of thing. They had a good name but were never the Gibsons or Epiphones of their time.
This one is more “the real thing” ....from the 30’s.
http://cgi.ebay.com/1930s-VEGA-CARVED-ARCHTOP-GUITAR_W0QQitemZ330162933588QQihZ014QQcategoryZ2385QQrdZ1QQssPageNameZWD9VQQcmdZViewItem
Best bet is to obtain a model number, either from inside, looking thru the bass-side f-hole, or perhaps from the butt of the neck if it’s a round hole, and google “Vega XYZ333 guitar”. You WILL find one or more out there.
Now, what do you suppose a 1939 F-Five Gibson mandolin would fetch (in excellent condition)?