Plasma's vault model allows participants to deposit stablecoins in exchange for the option to purchase XPL tokens in a future sale, with no immediate token issuance and full withdrawal flexibility

Quotes that support claims

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—and they can withdraw their funds at any time.

By depositing stablecoins into Plasma's vault, users earn the right to purchase the project's token, XPL, according to Plasma's website.