Let me start off my saying that I'm not a lawyer. I feel that this should be obvious, and that only a statement of the opposite would constitute a reason to believe otherwise. But I've been at the butt-end of our overly litigious American society's caprice before, and now I'm wont to take this and any potential legal threat somewhat more seriously. So to reiterate, I am not lawyer, and don't take what I say as legal advice.
Where are the internet's historical dressmaking archives and archivists? Surely, many dressmaking patterns and instructionals have entered public domain by now, and ought to be digitized and vectorized.
My verdict? While there are some freely distributed scans out there, available in low or medium resolution bitmap format files, the offerings are few. There is no centralization or organized volunteer effort to make them into vector graphics. Before I started my search, I think I half expected we'd be at the point of open source software, freely distributed, that used a form of pattern file with the addition of use measurements to generate printable, routable pattern graphics. I didn't find any such resources.
But what I did find was a closed-source community, slow to take advantage of technology, confused about copyright law and prone to spreading misinformation, that ultimately preferred hoarding designs to sharing designs. Most notably, I found a prevalent belief that a published and copyrighted pattern cannot be used to make garments for sale without explicit permissions or licensing agreements.
That's just not true. Copyright of a pattern extends to the pattern representation itself, not the concept of the pattern. This handout from copyright.gov explains it very clearly. "Copyright protection is therefore not available for ideas or procedures for doing, making, or building things; scientific or technical methods or discoveries; business operations or procedures; mathematical principles; formulas or algorithms; or any other concept, process, or method of operation." Also, "Copyright protection extends to a description, explanation, or illustration of an idea or system, assuming that the requirements of copyright law are met. Copyright in such a case protects the particular literary or pictorial expression chosen by the author. But it gives the copyright owner no exclusive rights in the idea, method, or system involved."
To understand how copyright relates specifically to fashion and garment making, the concepts of useful articles and separability are key. It turns out that those in high fashion, following in the steps of boat hull designers, would like to further protect their designs, but have not successfully done so yet. Even if the legislation they seek does pass eventually, that copyright would only cover a term of three years.
In summary, the whole idea that a dressmaking pattern is by default restricted to "personal home use" is standing on some very tenuous legal legs. Of course, if you have the time and money you can stretch the limits on what can you can register and legally defend as a design patent or trademark.
As part of the more general discussion about copyright law in the United States, I'd like to point your attention towards the Copyright Term Extension Act and 'the concept of 'forever less one day.' So mark your calendars for January 1, 2019 and expect a full show of force.