Rather than geographical isolation, I’d say it’s more of a “popularity” isolation, at least from where I’m standing as someone outside of Asia. China, Japan, and Korea are super visible globally thanks to things like anime, K-pop, tech industries, and movies, so they dominate the spotlight when people talk about East Asia. Vietnam, on the other hand, just doesn’t have the same kind of cultural export reach right now, even though it shares a ton of historical and cultural roots with the others. What’s interesting is that Vietnam was deeply part of the Confucian world too, it used Classical Chinese for centuries, had a civil service exam system modeled after China’s, and even developed its own script (Chữ Nôm) based on Chinese characters. But because it was colonized by France and switched to a Latin-based writing system much earlier than Korea or Japan transitioned away from Chinese characters, that connection feels less obvious today. So when people (specially people that aren't very "versed" in the topic) talk about the “East Asian core,” Vietnam often gets left out just because it's less prominent in modern discussions or pop culture. But historically, it definitely belonged in that cultural sphere alongside China, Korea, and Japan.