Ok, imagine I've deposited 100% eth, and vault is ... @beefyfinance

Answer: If the prices of ETH and WSTETH align in 6 months, you will not lose money apart from the APY, as long as the value of ETH and WSTETH remain the same. This is because, when you deposited 100% ETH, the Vault split it into 35% ETH and 65% WSTETH, so you still have the same amount of ETH and WSTETH, just in different proportions. If the prices of ETH and WSTETH remain the same, you will not lose any money. However, if the prices of ETH and WSTETH change in the 6 month period, then you may gain or lose money depending on the direction of the price change.

Why would you lose money?