Depends. Prefixes of Dutch names are lower case if preceded by another name, and upper case if on their own ("Van Vlack" versus "Wheeler van Vlack"). Prefixes of Flemish (Belgian) names never change case, so the name will either be always spelt as "Van Vlack" or "van Vlack", regardless of context.

I guess that in the US the Dutch/Flemish conventions do not apply, and the spelling choices will depend on the person/family.

I've seen it both ways. There's also the possibility of the lowercase form changing to uppercase with even more programmer assumptions about surnames always starting with a capital.

I'll bloody well decide whether I have capital letters in my name or not. That's sort of the point, here ;-)