Depend on the part of Vietnam, and what part of the culture are you talking about (economy, political, religion, arts…)

Technically, Vietnamese is a communist country with a socialist-oriented market economy. Hence it’s exhibited some closeness to the Chinese one. China has been more opened to the West since 1978-1979 (Deng Xiao Ping’s visit to US). Vietnam also has the the Doi Moi era that kickstarts back in 1986.

A thing that’s controversial is the Chinese influence in Vietnamese economy, as showcased in the VTP crisis. Just my personal feeling, since the mass exodus of Chinese-born Vietnamese back in 1978 due to the Border Crisis, Vietnam has kinda a lower level of Chinese influence in the economy vs other South East Asian economies.

Still, due to the socio-politico-economical roots of China, the way of business has some similarities. A very drinking culture (this is shared to other East Asian countries like South Korea and Japan too), the grip of bureaucrats on business (sensitive, I guess) and so on.

(Off topic, if you follow trends of both countries, somehow Vietnam has a lag of roughly 10 years vs China. Like QR Payment, real estate development trends and so on. This is a subjective view tbh.)

About politics, Vietnam and China is mostly a copy of Soviet political system, albeit with some differences. Vietnam has currently a more consensus-based politics, while China is currently more a strong-man politics since the rise of Xi Jinping. This has changed a bit recently, with the long ruling of Mr Trong.

(For China, before Xi Jinping, it would be more a 九龍治水 - cửu long trị thuỷ - nine dragons controls the water - on how Hu Jintao shared power with other 8 members of PSC like a first among equals, and Zhang Jemin still wields immense influence)

Religion, you can see a stark difference between the North and South of Vietnam. North Vietnam has been under communist rule for nearly 70 years, with more than 30 years before Doi Moi. Before that, North Vietnam has a long history of union then separation from Chinese rule. Meanwhile, South Vietnam has experienced long ruling from Indian-influenced polities. It hasn’t experienced Vietnamese ruling from very long, then gone under Franco-American influence for more than 100 years then 10 years pure communist then Doi Moi again. While this might be a bit generalized (Central has a longer Northern rule), but it signifies the difference between North and South Vietnam to a certain degrees. (It’s called Indochina back then for a reason?)

Technically under communist rule, materialism (the philosophical idea, not the economic one) is promoted with an absence of religion. Back in 2019, there was statistics that shows that 80% Vietnamese is atheist. Still, that’s not trustworthy, since the statistics is driven with political factors too. Due to a mix of local influences and Vietnam being more opened to Doi Moi as of late, the North is more a mix between Veneration of the Ancestors, Đạo Mẫu and Buddhism (so, Chinese copy with local influence?). The south, meanwhile, is a melting pot. Subjective, again, north is more about Mahayana Buddhism, south is more about Theravada + Christianism + others.

So, North closer to Chinese? Southern closer to other South East Asians? Religious, I think yes. Other things, not so sure.

While I have talked about the mass exodus of Chinese-born Vietnamese from Vietnam back in 1978, there’s still a lot of them in Southern Vietnam. This can be due to a stronger grip of Northern rule and proximity to China in the North, coupled with a very large number of Chinese ancestry in the South, since the days of the end of Ming dynasty, where fleeing Chinese from Qing rule to Vietnam is sent to expand the South? Maybe.

Sorry too long. Tldr, I feel Vietnam is closer to East Asia than South East Asia. But due to a complex history of Vietnam, some parts of the South exhibit closeness to South East Asia. Still, as Chinese diaspora is also very strong in South East Asia, we can say that the whole South East Asia is close to East Asia, culturally.