"At the trial, can Vault prove Kernel's oral commitment to repay the loan on or before July 1?
No, because such evidence contradicts the writing and is barred by the parol evidence rule
Yes, because the oral agreement was supported by an independent consideration. 
Yes, because the evidence of the parties' negotiations is relevant to their contractual intent concerning maturity of the debt. 
No, because such evidence is barred by the preexisting duty rule. 