You can unlock many different vehicles during the Japanesse campaign by obtaining certain medals:

Yamato Class Battleship: Obtain gold medal in “Invasion of Midway”.

Type B Submarine Equipped: Obtain silver medal in “Attack on Sydney's Harbour”.

Tone Class Heavy Cruiser: Obtain silver medal in “Battle of Java Sea”.

Shimikazi Advanced Torpedo Destroyer: Obtain silver medal in “Solomons Skirmish”.

Ohka Suicide Rocket Plane: Obtain a gold medal in “Invasion of Hawaii”.

J7W Shinden Fighter Plane: Obtain a silver medal in “Invading of Port Moresby”.

I-400 Submarine: Obtain a silver medal in “Hunt for the USA Hornet”.

H8K Emily Flying Boat: Obtain a silver medal in “Destruction of Zforce”.

Akizuki Class Anti-air Destroyer: Obtain a silver medal in “Saving the Shoho”.