If Scourge of Fleets returns creatures with toughness 3 or less, do my vampires at (4/4) get returned to my hand when Thirsting Bloodlord return?
Your Savage Gorger would remain on the battlefield.
Scourge of Fleets returns all applicable creatures at the same time. What creatures are applicable is only checked once, not continuously: when the ability resolves. What happens afterwards as a consequence of one or more creatures bouncing does not matter to Scourge of Fleets' ability, because its effect is a one-shot effect.

610.1. A one-shot effect does something just once and doesn’t have a duration. Examples include dealing damage, destroying a permanent, creating a token, and moving an object from one zone to another.