The words vain, vane, vein sound the same but have different meanings and spellings. Why do vain, vane, vein sound the same even though they are completely different words?
The answer is simple: vain, vane, vein are homophones of the English language.
Not yielding the desired outcome; fruitless: a vain attempt.
Lacking substance or worth: vain talk.
Excessively proud of one's appearance or accomplishments; conceited.
Any of several usually relatively thin, rigid, flat, or sometimes curved surfaces radially mounted along an axis, as a blade in a turbine or a sail on a windmill, that is turned by or used to turn a fluid.
The flattened, weblike part of a feather, consisting of a series of barbs on either side of the shaft.
The movable target on a leveling rod.
Anatomy Any of the membranous tubes that form a branching system and carry blood to the heart.
A blood vessel.
Botany One of the vascular bundles or ribs that form the branching framework of conducting and supporting tissues in a leaf or other expanded plant organ. Also called nervure.
Zoology One of the horny ribs that stiffen and support the wing of an insect. Also called nervure.
Definitions from The American Heritage® Dictionary of the English Language, 4th Edition and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.