From the released teaser and photos of Mersal, we had earlier predicted that one of the (flashback portion) older Vijay's sons might get separated from the family very early with or without his knowledge. And the boy who gets separated could grow up to be the magician Vijay. The other Vijay who comes in the Mersal Arasan lyrics video could be the doctor Vijay who lives with Kovai Sarala and Vadivelu.
 

What many would have failed to notice in the recently released flashback 80's promo video, is this young boy (check display image to see) who looks to be an acquaintance of father Vijay. When Thalapathy is seen in a red shirt arguing with the goons, that young boy could be seen in the background and the next frame, when he is fighting the goons without the shirt, the boy is again seen in the background holding Vijay's red shirt. The same boy could also be seen in the 'Aalaporan' lyric video. All these tell us that the boy looks close to Vijay and if you could connect him to our above stated theory, this boy could be the younger version of Vadivelu with whom the doctor Vijay might grow up.

This theory is just our assumption, and we could be wrong, but we felt it exciting to share with you.