i would take the frame and post to a bike shop and ask them to measure the seat tube inside diameter with their tool.

if i needed a 26.2 seatpost instead, i might think about filing 0.1mm from it. (0.1mm shaved all the way around equals 0.2mm in diameter.)

a lot of new posts are also way too long for vintage bikes, so you could cut it to size and try it in the frame again. you only need about an inch of post below the seat lug.

also, old bike frames develop a lot of scale inside the tubes that you may try to file or sand away.