"Vietsub" refers to Vietnamese subtitles. For portable viewing, these are often hardcoded (burned-in) so they display correctly on any mobile video player app.
Vietnamese fans typically seek "Vietsub Portable" versions for several reasons: fate strange fake vietsub portable
The war is split between "Fake" Servants summoned to agitate the leylines and "True" Servants brought in for the actual battle. "Vietsub" refers to Vietnamese subtitles