T-54M, T-90S, T-90SK, most modern MBT main battle tanks in service in Vietnamese army.

According to VietDefense’s Facebook page, the T-54M and T-90S/SK are the two most modern main battle tanks in service with the People's Army of Vietnam. Having first arrived towards the end of 2018, the majority of the 64 T-90s are operated by the PAVN's 201st Tank Brigade.
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T-54/55M and T-90S of the Vietnamese army (Picture source: VietDefense FaceBook page )

The amount of T-54Ms or the units they are serving with isn't publicly disclosed information, though it's worth noting the process has been going on for a few years. Although there was a minor pause in the upgrade process due to Covid-19 and budget shifts, Factory Z153 has since resumed the modernization program. The newest example was an unnumbered and unpainted T-54M that was spotted returning to Z153 after field trials the other day.

In April 2018, PAVN was updating a number of legacy T-54/55 tanks to the T-54M3 standard. The Vietnamese army had launched a program to upgrade its fleet of T-54/55 tanks. He adds that "several dozen" tanks will be modified. The modernization of the whole fleet of T-54/55 MBTs is unlikely due to financial constraints. The T-54M3 has been fitted with additional Super Blazer explosive reactive armor that protects the tank's frontal arc and sides. The vehicle has retained the organic D-10T2S 100 mm rifled gun of the baseline T-54/55. The T-54M3 has also received a new sighting system with infrared channel and a meteorological sensor. The tank's armament suite has been reinforced by a 60 mm mortar installed in the left part of the turret. The organic DShKM 12.7 mm heavy anti-aircraft machinegun has been retained also. All the new components of the T-54M3 tank are supplied by Israel's Rafael company.

According to the 2016 annual report of Rostec`s Uralvagonzavod corporation, the manufacturer of the MBTs, a contract with the Socialist Republic of Vietnam for 64 T-90S and T-90SK (K for Command, Komandniy) MBTs was signed in 2016. The Socialist Republic of Vietnam (SRV) is reported to have received the first batch of 32 MBTs in late December. The PAVN now operates T-90S, Type 59, T-54/55 (including T-54M), T-62, PT-76B, and even WW2-era T-34/85 tanks.

T-54/55M and T-90S of the Vietnamese army (Picture source: VietDefense FaceBook page )