What kind of surfaces are semi-vitreous?
When specifying porcelain for tiles or work surfaces, one can have, in ascending order of porosity, non-vitreous, which holds the most water, semi-vitreous (in the middle), and impervious, which holds little or no water. In view of the stain-removal context, I would interpret "at least semi-vitreous" to mean "semi vitreous or impervious".