vacuum advance miss

[JFORRESTERSR]

I have a 1972 k5 blazer with newer engine, not sure what year but it was blue so I am guessing late 70s. It has the hei distributor with the coil on top
heres my issue it has a miss at idle but when I disconnect the vacuum
advance it goes away. My first thought was a bad vacuum diaphragm
but then I put a vacuum pump on it and it held so I am not sure where to go
i did a tune up when I got along with a 1000 other things and the old looked like it had not been changed in 100 years my first thought is to replace the whole distributor there pretty cheap but I thought I would ask around first in case it was something stupid I was missing

thanks for the help