The county of "Ascadian Isles", where Vivec is located, has only a placeholder flag, even though the rest of Vvardenfell has set flags and, in fact, a flag has been created for it and is in the GFX folder. The problem seems to originate that the new flag has been labelled c_vivec, while the county's actual name is c_ascadian, which has a generic placeholder flag assigned. I copied the GFX for c_vivec over c_ascadian and the problem was solved.
Also, this appears to be a problem with the base game, but I had an annoying incident where my dynasty's Coat of Arms changed. I was playing as a ruler designed Ashlander in Vvardenfell (where I've since kicked out the Temple and established the Kingdom of Vvardenfell as an independent power), but I had an incident where a cabal set itself up in my capital, Maar Gan. I immediately sent my court wizard to root the cabal out, but a year later before he could do anything I got an event where the cabal had 'taken root' and my capital and character suddenly converted to the Sixth House cult! My dynasty's coat of arms immediately changed to a random design, then changed to another random design when I used a console command to make myself a Nerevarine worshiper again. Sadly I've had this same bug in the base game, so I have no idea if you guys can find a way to prevent it from happening...