I've been reading through the Ten Books on Architecture for a project, and I've begun to wonder: does the book serve a descriptive purpose, wherein Vitruvius describes how architecture is "done" around the empire, or does it serve a more admittedly prescriptive purpose, in which he is writing to establish how he believes architecture ought to be done? If it's the former, what level of standardization was there throughout the empire for Vitruvius to catalogue? If it's the latter, how much impact did Vitruvius have among his contemporaries (as it's clear he's had an impact on his successors). For instance, when he discusses the proportions of an atrium in a house, are such proportions gleaned from observation of many homes, or are they the set of proportions he finds most pleasing and believes should be the standard?
In short, it's both descriptive and prescriptive as far as I can tell. If you read his preface (I have the version translated by Morris Hicky Morgan) he writes a couple of key things.
...you were giving your attention... to the providing of public buildings intended for utilitarian purposes, so that not only should the State have been enriched with provinces by your means, but that the greatness of its power might likewise be attended with distinguished authority in its public buildings, I thought that I ought to take the first opportunity to lay before you my writings on this theme.
Shortly after this he writes
I have drawn up definite rules to enable you, by observing them, to have personal knowledge of the quality both of existing buildings and of those which are yet to be constructed. For in the following book I have disclosed all the principles of the art.
Certainly he's writing his own beliefs about architecture, but my understanding has always been that it's based on his observations of architecture within the Empire. In a way he's just codifying what he's seen and what he feels is best, but to me, this certainly has to be based on established architectural ideas.
Hopefully there's someone who can answer the questions about standardization. I think it would largely have to do with how disseminated the work of Vitruvius became, which unfortunately falls out of my expertise. I would hazard a guess that it certainly played a role to some degree, but I can't say how much. That's, of course, only a gut feeling.