Vault positions are represented by rfTokens.
FTMScan: rfWFTM token issued by the WFTM Multi-Strategy Crypt
When you deposit assets into a vault, you will receive rfTokens specific to that vault, proportional to your investment size (see Vault Shares below). rfTokens are essentially coupons the smart contracts issue to track your investment. When you withdraw, the contract needs your rfTokens in order to return your funds. You do not need to add them to metamask or send them anywhere. You never need to interact with these rfTokens directly. The Reaper Farm contracts will retrieve them from your address automatically. If you lose your rfTokens, the Byte Masons cannot help you.
Because rfTokens represent your vault position, they are treated as an interest-bearing asset. In most countries this is tax-efficient because you no longer have to record claims and trades when farming manually.
Vault shares can be seen when withdrawing from a vault.
It may be confusing because the number of shares you have will be different to the amount of assets you deposited, but vault shares indicate how big of a share of the total vault your deposit entitles you to. When you withdraw assets, you are reducing your share of the vault and thus your total vault shares (same goes for deposits). They are simply a representation of your stake in the vault.