Sizes are not the same today as they were 80 or even 40 years ago. So the safest way to pick the right sie is to ignore the size number completely. It's just a number and means nothing. When looking at a vintage pattern, first measure your chest or bust. Measure the largest point on your chest or bust and compare it to the measurements for the pattern you'd like to get. Then purchase the pattern that is right for your measurements. That's what's important.
But you also have to understand wearing ease. This is the difference between the garment measurements and the body measurements. To give an example of this, a pair of pants with a 32" waist might actually be designed for someone with a 30" waist to allow for comfort and movement. Vintage patterns have less wearing ease than their modern counterparts. You may have to add ease to make the garment comfortable. Consider sewing a test garment out of a cheap fabric to see if you'll have to make some adjustements (or even use a larger size).