If my foot measures 10x4" the charts say I'm a 9 but a lot of the shoes I'm looking at (NOS/vintage) have 11-12" insoles for size 9. How big should the insole be in relation to my foot? I'd think half an inch bigger, max but that would mean I'm looking for size 7's almost. I'm really confused here
You're asking for trouble if you base your shoe purchases off your foot measurements. Every maker uses different lasts for their shoe offerings. All of them fit the foot in a different manner. You may wear a 9Din last #1 and a 10C in last #2.
If possible, it's in your best interest (both financially and foot comfort) to try the shoes on in person before purchasing.