Answer:
1. SELECT VendorID, MAX(InvoiceTotal) AS InvoiceMax
FROM Invoices
WHERE InvoiceTotal - CreditTotal - PaymentTotal > 0
GROUP BY VendorID
2. SELECT SUM(InvoiceMax) AS SumOfMaximums
FROM (SELECT VendorID, MAX(InvoiceTotal) AS InvoiceMax
FROM Invoices
WHERE InvoiceTotal - CreditTotal - PaymentTotal > 0
GROUP BY VendorID) AS MaxInvoice