The Adityas and Vasus refer to two different groups of sons of Kashyapa and Aditi, and Indra is an Aditya and not a Vasu. But Indra is called Vasava for multiple reasons. First of all, because he is the king of the gods, he is also the lord of the Vasus. Second of all, it's because after his famous defeat of Vritrasura, he was considered the vasu or treasure of the gods; here is what this chapter of the Shatapatha Brahmana:
When Indra had hurled the thunderbolt at Vritra, thinking himself to be the weaker, and fearing lest he had not brought him down, he concealed himself and went to the farthest distances.... Agni of the deities, Hiranyastûpa of the Rishis, and the Brihatî of the metres, set about searching for him. Agni discovered him and stayed with him (as a guest) that (day and) night. He (Indra), namely, is the Vasu of the gods, for he is their hero.
Here is how Sayana explains the use of the term Vasu:
That is, as would seem, the benefactor, or the treasure (dhanarûpa, Sâyana) of the gods. Indra is the chief of the Vasus. Indra being so beneficent and important a personage, it was, according to Sâyana, worth Agni's while to stay with him.
By the way, when the Ramayana calls Indra the elder brother of Vishnu and when the Bhagavad Gita says "Of the Adityas I am Vishnu", they mean Vishnu's incarnation Vamana the dwarf, who was the youngest son of Kashyapa and Aditi. As I discuss in my answer here, Vamana in particular is called Vishnu because when he grew large he pervaded the Universe. When you are looking in the Vedas for references to the god that we now call Vishnu, you should look for "Narayana" rather than "Vishnu", because references to the name Vishnu generally focus on Vishnu's Vamana incarnation.