Well, kind of the other way around. If the engine vacuum were low, it would be due to the engine not running correctly, and so much so that I'd expect you would notice it not running well. It would either be running roughly and shaking or the engine speed would be quite low at idle. Does your Liberty have a tachometer in the instrument cluster, and if so, does the engine idle at a lower speed when it seems low on power versus when everything seems fine?
-Rod