Good thread
Even if a vintage bass doesn't increase in value at a rate that makes it a good investment at the moment, it will almost certainly hold its value better than a bass bought new.
I have bought a number of boutique basses over the years and, much like driving a new car off the forecourt, they have always lost a large chunk of their value as soon as I picked them up - the one exception to this rule are Wals! - so now I'd rather pay a decent price for a used instrumet that I know is a fantastic player. This applies to proper vintage basses as well as instruments of more recent construction.
My 1970 fretless P bass is a good example: it's an absolute tone machine and is just a joy to play. It wasn't cheap, has plenty of "mojo" and isn't 100% original as the original pots had been removed (I put a vintage set back in though ), but it will hold value.
Happy hunting!
Gareth