[quote name='clauster' timestamp='1370774691' post='2105284']
If you really want to buy a bass as an investment, look for basses that are under-valued at the moment.
[/quote]
Would be inclined to this advice, if thinking short- to medium-term. In buying a vintage Fender now you are buying high in the hope of selling even higher; you could probably better that return by finding something currently undervalued and which might increase in value - at least in percentage terms.
In the long-term you're banking on all kinds of trends, and perhaps a vintage Fender would work - if the cachet remains, amidst a whole host of provisos; and possibly carry more certainly than some of the more obscure, or lesser, brands over that time frame.7
No guarantees, of course, as with all investments. Just because Fender's have increased in value doesn't mean they always will, or at such a rate, etc. The world could go to sh*t and it could just be worth firewood