I actually have several vintage size 12 and 14 clothing that fit me and I wear between a modern 2 and a 4.
I used to tell people they had to just try it on - you can't go by the size. Of course someone who normally wore a size 4 wouldn't believe you when you suggested they try on the size 14 - most thought it was just a sales pitch but then they would come out of the dressing room and it was too small.
I would size the clothes by trying them on myself and then I would put on the tag "approx. size 6" or whatever my best guess was based upon whether they were too big or too small for me. I got really good at it.
But trust me, women do not like being told they are several sizes larger than they really are.
By the way, I never saw a vintage tag with the size 2, 4 or 6. I think they started at 8 or 10.