Vowels in Spanish are pronounced quite differently than in English. Each Spanish vowel represents a phonetic sound, and can include accent marks that specify the amount of stress to apply when pronouncing a word.
The Learn to Speak Spanish blog has a good post up about how to pronounce vowels in the Spanish language properly. If you want to develop a good accent as you learn the Spanish language, definitely give the post a look.

http://learntospeak-spanish.com