Its flag is the same as the Army's but with the inscription below "Biên Pḥng Việt Nam" (Border Defence Viet Nam) as seen here. Esteban Rivera, 13 February 2016
Viet Nam People's Border Defence Force (Vietnamese: Bộ dội Biên Pḥng Việt Nam, BDBP) (Border Defence Force Viet Nam) is a Branch within the Viet Nam People's Army. It was established on 19 November 1958 by Politburo of the Communist Party of Vietnam which decided to unite national defence forces and army units whose mission is to protect domestic, border protection, beach, boundary and other armed forces in charge of protecting the domestic and border guards, under management by Police Department, named "Guard Forces." The "Guard Forces" are divided into "Border Guard" and "Homeland Guard."
Source: Wikipedia article Esteban Rivera, 13 Februry 2016
A photograph online shows a green flag with a symbol and text on the bottom being used by naval units of the Border Defense Force (green is a traditional color of border guard/defense units from the former Soviet Union). Unfortunately, I could not get enough details of the flag to make a reconstruction image. Zachary Harden, 14 February 2016
image posted by Jason Saber, 12 May 2021
Sorry to bombard you but things keep appearing on the FOTW Facebook page and I hate to see them disappear without recognition!
I checked your Vietnam page but could not see this there. It was posted [on Facebook] as "Possible maritime flag of the Vietnam People's Army Border Defense Force." I have no further information or confirmation about this but thought you would like to see it. Jason Saber, 12 May 2021