"From the most common of items to the most precious of gems, all are stored within your Royal Vault!"
Every item stored in the vault takes up one space, regardless of how large or small it would be in "real life".
The capacity of the vault begins at 50 when you start the game. You may expand the capacity by 10 items at a time by using Silver Vault Hinges at first. Soon, you will also need Golden Vault Padlocks. As your vault grows, you will need more and more of these with each expansion. To expand beyond a capacity of 250, you will also need to use iron ore and stone.
In addition to the cost of using the various items to expand, the larger vault also increases the number of items that Liang will require for his largest purchases.