I think it needs to be cleared up whether it simply won't idle below 950rpm in gear(as in, that's as low as it can be made to go), or......it doesn't like to idle below 950rpm in gear.

Frankly, with a 300deg cam, 10" of idle vacuum seems really good to me.
And, with 28deg initial timing......that will tend to make the motor idle at a higher rpm, and probably helps explain the (higher than I'd expect with a 300deg cam) vacuum reading at idle.