Our idle spec is 750 rpm when warm. It's totally controlled by the PCM and there's no way to mechanically adjust it. I'm guessing if you're more than 100 rpm out of spec there's something that needs attention. The PCM is trying to keep idle at 750. I'm not saying it's a huge problem. Sometimes these things are hard to track down and you may make it worse.
I just took this pic of mine a few minutes ago idling in park while warm. It's purring like a kitten at 750 rpm. If my engine with that many miles on it can idle correctly, then there's no reason why every other VX engine can't do the same.
Mark Griffin