If Prodigal now sues Vault Savings and Loan for $10,000 plus accrued interest, will the action succeed?
No, because Prodigal's right, if any, to the funds on deposit was effectively abrogated by the second Mater-Vault deposit agreement
Yes, because Prodigal was a third-party intended beneficiary of the original MaterVault deposit agreement. 
Yes, because Prodigal was a constructive assignee of Mater's claim, as depositor, to the savings account. 
No, because Prodigal never obtained possession of the passbook. 