Is the Flash invincible? What other character an travel near the speed of light?

The Flash is not invincible, though because he can vibrate things close to him to specific frequencies, certain writers explain that he can shield himself in this way. But other writers take advantage of the fact that he is travelling so fast to make him more vulnerable should he be struck while running, because the impact is so great. There are lots of Flashes and Flash-family characters that all tap into the "Speed Force" in the DC Universe. Check out the recent FLASH: REBIRTH for insight into those characters and their powers.