A vintage accordion is a romantic object: ornate, weighty, history in mother-of-pearl. And the prices on old ones can look astonishing next to a new instrument, which is exactly the problem. The cheap vintage accordion is one of the great traps in music buying, because the most important parts are hidden, perishable, and ruinously expensive to fix.
What you cannot see and cannot easily afford
An accordion is a box of reeds, leather valves, and bellows, all of which age. The leather valves dry, crack, and stop sealing; the bellows develop leaks; the reeds can corrode or fall out of tune; the wax that holds reeds can degrade. None of this is visible from the outside, and a thorough restoration by a specialist can cost more than the instrument is worth — sometimes far more. The pretty case tells you nothing about the health of the lungs inside.
When the bargain is actually real
The vintage buy makes sense in two cases: when it has been recently and verifiably serviced by a known repairer (ask for the history), or when you are buying knowingly as a restoration project and have priced the work, not the instrument. A cheap, unserviced accordion bought hoping it is fine is a gamble where the house holds the reeds. A cheap one bought with eyes open about the work ahead can be a joy.
The honest advice
If you just want to learn and play, a modest, working, modern or recently-serviced instrument will frustrate you far less than a gorgeous, silent antique. Buy sound before you buy soul. The romantic vintage accordion can wait until you know enough to judge its lungs — or to love it as a project rather than expect it as a bargain.