Even within predominantly German speaking areas there were villages that were majority Czech. Libníč was one of these. In 1910 for instance there were 348 Czechs and 2 Germans living there.
As for "vulgo" or "vel" names: Most of these came about when somebody with an established family name took over a farm with a farm name. That happened usually either by marrying the inheriting daughter or by purchase. For instance one of my ancestors, Jan Wawra (Wawra-2), bought a farm in 1701 from one Jan Klabauch and from then on was known as Jan Klabauch. This was a strait forward change of name that stuck. But quite often that did not happen and people were known by both their original family name and the farm name. In those cases they were often recorded as "x vulgo y". But your hunch is right, it was entirely up to the knowledge of the record taker and many families ended up with records naming them "x", "y", "x vulgo y", or "y vulgo x". If the dual naming persisted you would later encounter fixed combinations, I have people in my family with "x v. y" as family name consistently in all records, from birth/baptism through marriage, address registrations, and as mothers of their children and death. That's one of the reasons I don't like uniform rules when they deviate from what is in the primary records.