The Vault rewards you with a free skin after you complete 4 trades over $10 each.

Each trade over $10 adds to your Vault. The amount added is a small part of the trade. For example, trades of $80.39, $48.72 and $60.28 ($189.39 in total) give a Vault worth $0.94.
After 4 trades, your Vault unlocks.
Choose a free skin worth up to your Vault's total.

If the skin you choose costs less than your Vault's total, the difference is added to your balance.

Where can I see my progress?

On the Trade page, the Your Vault panel shows how many trades you have left. Click Preview Reward to see what you can currently receive.