An alliance is a dangerous move, for both Vietnam and Japan.
Firstly, the US will not allow Japan to ally with China. Their troops in Japan are not for show. Japan is an important base for the US across the Pacific. They will not accept Japan "two timing".
Secondly, it's personal opinion, Vietnam and China could have had an alliance. In fact, before China first publicly release the 9 dash lines map and claimed most of SCS for itself, things went pretty well between Vietnam and China. The completion of land border marking was a great success for both countries, and our media reported all day about "4 diamond words, 16 golden words" about the relationship with China, even though Vietnam govt did "return" some of our land to China. And then China messed it all up with the map, then the rig. So now, an alliance is next to impossible. The people will not allow it.
Personally, I think the idea of an alliance over East Asia is quite similar to Japanese "Great East Asia Empire" in WW2: sound good but impractical. We all have a long history of fighting each other, and the difference in ideaology, culture and traditions will certainly cause discriminations and mistreatment among the people. Look at Hongkong now. They are Chinese people, but prefer democacy over communist party rule. And the mainland Chinese already start talking bad about them. That's just inside China alone. Vietnam also have the same problems and I am sure every country has their own internal issues. Now imagine that happens between countries, and politicans get involved. Boy oh boy, I'm sure there will be a war, instead of an alliance.