Yup, it is creating a vacuum leak. Same with the dipstick. Mine does that as well. Of course, why would you want to remove the oil filler cap while the engine is running anyways. I am not a big fan of having oil all over the place.
The engine dies because of the way the VAF works, not solely because doing so creates a vacuum leak. The VAF operates on the principal of a pressure drop across it, which causes the plunger to move back, allowing air to move through and into the engine. If you take a pressure reading on either side of the VAF with the engine idling, you'll see that the "throttle body" side is a bit lower than the "air cleaner" side.
When you open the oil cap, you're allowing air into the engine. On the top of both valve covers are tubes that connect to the big breather tube on the back of the throttle body elbow. Air normally flows into that breather, through the engine and is drawn out via the PCV valve into the intake. All of this air is supposed to be all measured by the VAF; both the air entering the throttle body and the the air bypassing it and going the PCV route.
When you open the oil cap, you basically vent the entire system to the atmosphere, including the volume behind the VAF. As a result, the VAF will close and the PCM sees this as a reduction in airflow. It'll reduce the fuel amount, the engine will stumble and likely stall. Not always, but most.
My V6, with the MAF in place the VAF, runs quite happily with the oil cap off, spewing oil over everything in the process.
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