vantage > profit, benefit none > anyone
5 And helpless hap it boots not to moan.
helpless hap it boots not to moan > [it is of no avail to bemoan an unavoidable occurrence]
6 Dead is Sansfoy, his vital pains are past,
vital > [of life, worldly]
7 Though grieved ghost for vengeance deep do groan:
grieved > [his] injured
8 He lives, that shall him pay his duties last,
He > [The Redcross Knight] duties > debts
9 And guilty Elfin blood shall sacrifice in haste."