Is it possible it switch the flat handle bars of a vintage bike to drop handle bars?
It is
possible , as said, not cheap , unless you get another bike to cannibalize, for cheap/free ..
old guy, I now like figure 8 bend Trekking bars , as a simpler swap from straight bars ,
because of the ease of re using the contol levers off the staight bars .. is there .
Most LBS can get some . prefer online/mail order ? under $20..
XLC Multi Functional Trekking Bar, 580mm, 25.4, Black