Originally, I was going to create an adapter that would go from the original vacuum hose fitting to the sander. The total adapter chain would have ended up being about 6 inches long in this case. Since the adapters are all rigid, it would have really gotten in my way to have 6 inches of rigid plastic between the sander and the flexible hose.
I realized that the first vacuum hose fitting threaded on the flexible hose with a reverse thread. I had just figured out how to make threads in Solidworks, so I was able to make the thread pretty quickly.
I knew the part would have significant stress on it as it dragged the relatively heavy vacuum hose back and forth as I sanded, so I printed it with solid infill. Ultimately, the entire adapter chain ended up being about 2 inches, so I ended up with a much better design.