I can't give a greatly comprehensive answer because we simply don't know if there aren't things like Fechtbucher in between Vegetius and Ms. I.33 that we just haven't found, or if they just weren't written.

I can answer a bit about the context of the Fechtbucher corpus that starts with I.33, and unsurprisingly, it has far more to do with culture than it does with sophistication (or lack thereof) of fencing techniques.

As a quick comparison between Vegetius' tract and the anonymous author of I.33, it's important to point out that De re Militari is written about organizing, training, and supporting armies and not about training specific skills for individual swordsmen. As such, it reflects the medieval and early modern military tracts like those written by Fronsperger and Wallhausen, rather than the corpus of fencing manuscripts and treatises such as those written by Fiore and the Gesellschaft Liechtenauer.

Ms. I.33 also needs to be put in its context, but even that's not easy. The author is clearly a monk, and the images not only depict monks as students and fencers, but also a figure thought to be Saint Walpurgis - the manuscript itself is often called "The Walpurgis Fechtbuch" - and it's unclear, at least among the Historical European Martial Arts community, whether the techniques contained in the manuscript are meant for personal, earnest combat or merely for fun and exercise. The reasons for this ambiguity are complicated, suffice it to say that, even though the Walpurgis Fechtbuch is the earliest manuscript we're aware of that forms a part of the corpus of Fechtbucher, it doesn't really fit very neatly into a cultural context of personal duels for honor or battlefield combat that characterize the later manuscripts.

One thing that gives some insight into Ms. I.33 is the fact that the anonymous author is (at least very likely to be) a monk, and monks were very clearly a sub-society in medieval culture that was literate, when the case may not have been true of other subcultures. Reading and writing and scribing in general were important parts of a monk's daily life, and so fencing, even if it was just a pastime done for fun, would have been a target of scribal attention just as any other aspect of monastic life would have been. I unfortunately can't comment broadly on monastic life and the pre-printing press writing culture (maybe another historian can).

There's simply too much that we don't know to really say for sure. What we can be somewhat confident about, however, is that the writing of fencing manuscripts and treatises reflect generally the state of literacy and the culture of writing and printing in Europe as a whole, with a few deliberately obfuscated treatises extent before the advent of the printing press, and then an explosion of material following after, until the "dueling craze" of the early 17th century and beyond. That a monk, or someone closely related to monastic life, was one of the first to write a fencing manuscript, should not come as much of a surprise.

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all that we really know of Ms. I.33 is summarized here, but unfortnately it's really not much, and it desperately needs more treatment of the context of written instruction manuals in the 14th century.