Whats the most effective treatment to itching and ance in skin of external anal? ,whats the effect of vaseline on it?!

Depends on cause: I would encourage you to have an inspection by a physician. Itching could be an allergic reaction, a hygiene issue, hemorrhoids or other GI problem including infection. Vaseline is generally safe as it hydrates the skin and is non-reactive. Consider using wet wipes after having a bowel movement. If you have acne nearby, consider over the counter washes.