A win for Japan will all but guarantee their spot in the top two. Spain's fate could end up being sealed by this very match depending on how they perform against Finland.
If Vietnam continue to play so passively like yesterday, it will be probably a 4-0 for Japan, and they will advance further (RIP Spain). I don't doubt that Vietnam will try their best to the very last moment, but if they don't change something it will be hard for them. You have seen Japan's aggressive playstyle and it will be a big trouble for the Vietnamese team if they continue to play passively.
It's kinda sad for the SEA scene though because this Vietnamese team wrecked all Singaporean teams recently (like, they 4-0'd them all), and Singapore will be up against Canada and Russia, so, RIP. Thailand please carry us.