w/ the vac line pulled you're getting a larger vac leak and also richer mixture b/c the fpr is at atmospheric pressure (wot fuel pressure). when you plug the vac line to the fpr it'll lean it out at idle b/c it's dropping fuel pressure. also you're getting less air in w/o the vac leak.

what ecu do you have running it? you should be able to raise the idle screw to fix the idle, but if the iacv is blocked or not working you'll have the issue you are describing as well. what tb, gasket, and intake manifold are you running?

any cel?