Plasma’s vault model allows users to earn yield on their deposited stablecoins while retaining the option, but not the obligation, to purchase XPL tokens, with full withdrawal flexibility at any time

Quotes that support claims

They can withdraw their funds at any time. They'll also earn yield on their deposited tokens in the meantime.

By holding funds in the deposit vault, users earn the option to purchase a share of tokens in the future sale, but are not obligated to do so