In most cases, vintage bike frames are going to be made of chromoly steel. If these frames have no issues with rust, then they will continue to last for many more decades, if rust does not become an issue. The only thing then to worry about would be the components of these vintage bikes. For most brands of vintage bikes there are enough spare and aftermarket parts that you shouldn't have to worry about replacing these components for many years to come. The only exception to this general rule would be the Peugeot brand. Therefore, I would suggest you think twice before purchasing a Peugeot of any kind. Peugeot parts are scarce!