We can fetch the bill credit from a vendor payment transaction using suite QL.
function getBillPaymentCredits(billPaymentId) {
var queryResults = query.runSuiteQL({
query: `
SELECT
Applied.nextdoc as billCreditId
FROM
AppliedCreditTransactionLineLink as Applied
WHERE
Applied.paymenttransaction = ${billPaymentId}
`
});
var results = queryResults.asMappedResults();
return results;
}

