Vedas do not talk about avatars. When and why this concept got introduced in Hinduism?
When avatar got introduced?
First avatar appeared when Brahma was struggling to get earth back in this creation cycle. Lord Varaha appeared.
Why avatar got introduced?
The purpose of avatar is to restore dharma.
Bg 4.8 — To deliver the pious and to annihilate the miscreants, as well as to reestablish the principles of religion, I Myself appear, millennium after millennium