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How many miles on this vehicle? Is most of it's use in town? If so, it sounds like a possible plugged catalytic converter.
Let me know and I will try to help you isolate so you know for sure.
My guess, without being there is that the catalytic is plugged up. The signs you are showing me are classic to this. Generally, after the engine is warm you can open the throttle wide open and the vacuum on the engine will drop to almost zero. If that is the case, unhook the exhaust between the muffler and catalytic converter and I think you will find that it runs fine.
If not, left me know. Otherwise replace or eliminate the catalytic converter depending on what State you reside in.
Be sure and let me know if that eliminates the problem. I think that is what is wrong. The real test is the vacuum gage reading under a load.