And had Vietnamese used Chinese characters to this day, learning Chinese and vice versa would be easier for those with a full education.
Not using hanzi makes learning language at the elementary level much easier, and many countries dropped use of them to improve literacy rates during their process into developed countries. But once your country is developed and education is good, hanzi has the advantage in that its fairly uniform and lets readers quickly understand new words even in different language.
That's because you picked Japanese as an example. If I pick Vietnamese that's the opposite, the Chinese characters are hiding the similarities of both languages to a non-initiate Vietnamese speaker. Japanese is maybe the only language which benefits from the current writing.