Upvote:2
I was told by a fellow Australian living near Phong Nha (about 40km from Dong Hoi) that there is an immigration office in Dong Hoi, but they do not do visa extensions. I did not have the patience to look for alternatives in Dong Hoi.
When we were passing through Hue (to the south) we found plenty of places who would arrange the extension for us, so that is likely the nearest place to reliably get it done. However, please note that all of the places we inquired at told us that the visa would not be extended, but replaced. Apparently they cancel your original visa and give you a new one valid for 30 days from the date you apply for the extension, not from the date of expiry on your first visa.
Upvote:4
No, you will need to send your passport to Hanoi, Ho Chi Minh City or Danang to have your visa extended. You can also find a travel agency in Dong Hoi who can assist you in extending your visa.