It does sound more rough and archaic, far less "sweet" than a violin. But that has a lot to do with the setup, less with the body shape. No soundpost, but quite long bass bar. Then the bridge style makes 50% of the sound. This crown shape in the pictures is really harsh. I've replaced it with a rebec style arch bridge (sadly have no photo of it atm), which is better but still distinctly medieval. I also fitted and tried a modern violin bridge, which makes it sound pretty close to a violin!

Please note that vielles are not standardised like modern violins. It's really any violin/viola sized bowed instrument with flat or just slightly arched back and top. This is really just my design loosely inspired by old paintings and modern makers.

Also the hair on the bow has a huge influence on the sound. I rehaired a baroque bow with black horsehair, which is very coarse and gives a rough sound.

Also seeing a pretty clear linguistic link between "vielle" and "fiddle."

In german it's Fidel (like Castro). And I've been told it's distinct from "Fiedel" (long I like "feeling"), which describes a cheap but modern violin.

I wonder if the "rough" sound you describe would be at home with various European and North American folk traditions.

Pretty much. Tbh. it was the soundtrack of witcher 3 that got me into folk and early music and making my own instruments.

One step further would be to make an instrument with horsehair as strings, like tagelharpa or кобыз (kobyz)