Why does the Reverse-Flash vibrate?
Eobard Thawne chooses to vibrate for exactly the reasons you just said: to look scary and ominous. Additionally, he also does it in order to mask his identity- which Barry himself also does when he talks to Iris in his alter-ego. The red eyes also are just for show, and are there only just to make him look scary. Why is Reverse-Flash eyes red? It is shown that the eyes of Nora West-Allen the daughter of Barry and Iris in The Flash, Season 5 Episodes 19 & 20, glow red.