Given Apple have complete control with their OS of how the dynamic foveated rendering system works, if they particularly wanted a screenshot feature to be included - reasonably likely for marketing purposes, it seems to me - then surely it wouldn't be too hard to code it such that it renders a second version of the GPU's output to generate the screenshot, when asked to by user, which has even pixel density. (Or perhaps screenshots getting some sort of "bokeh" for the first time, highlighting whatever the user was looking at when taking the screenshot, will turn out to be a desirable feature.)

Not having buttons obviously doesn't prevent there being a screenshot function, just as lack of screenshot button on iPhone hasn't prevented it from taking screenshots, and just as the VP's lack of buttons doesn't prevent users from interacting with it at all...