I need a way to convert a Postscript file to XPS. I'm using VB.net so any method with an API or command-line would be fine. As far as I know Ghostscript can't do it but I would love to be wrong.
Ghostscript can do it, but there are severe limitations. You can use the xpswrite device to produce XPS as an output (in a similar fashion to pdfwrite to produce PDF).
However the xpswrite device is much more primitive, it hasn't had much attention paid to it over the years given the lack of interest in XPS. Some pages may emerge being rendered, and some fonts may be bitmapped (XPS only supports TrueType fonts, PostScript has a much wider range of fonts).
Etc, etc.
However, since the task will require a full PostScript interpreter I suspect your only other choice is to spend considerable sums of money. I think Global Graphics might have a commercial solution for this, there may be other suppliers but none of them will be open source (or, indeed, cheap).