Which Devas in Vedas (mainly Rigveda) are never referred to as Asuras and similarly which Asuras in Vedas (mainly Rigveda) are never referred to as Devas? I need references only from Vedas and nothing else.
A couple of points from Sanskrit, to avoid pitfalls when approaching Deva, Nara, and Asura. Not all of them are from the Vedas, but I feel it's important to consider them before we fall into the same pitfalls over and over again:
In general one can only be Deva, Nara, or Asura. Technically there has to be an in-between form for there to be a change between them, but this is rare.
Sometimes individuals change whether they are Deva, Nara or Asura. Thus, what they are referred to changes depending on time.
Deva (possibly others) sometimes shapeshift, which changes their Deva, Nara, Asura classification.
When in a compound Deva, Nara, and Asura more often than not take their word meaning and not whether they are a Deva, Nara and Asura. Thus Deva can have Deva, Nara, or Asura in their names and the same is true for Nara and Asura.
In Tamas scripture "Viṣṇu," is used for all his forms, not necessarily Viṣṇu Vaiṣṇava, without it being obvious which one it's talking about. Maināka Parvata being the most obvious example of the Tamas scriptures doing this, as his family relationships make him out to be the one called Viṣṇu in many stories. "Indra," has the same complication.
Okay. That technically does not answer your question but should avoid wasting time for anyone with an actual answer.