Interesting Observation

I'm trying to learn American English. Recently I noticed that a lot of Americans use the same vowel "a" in words like "father", "mall" and "talk", "thought". I wonder if it's okay for me to pronounce these words with the same vowel? There are far less vowels in my native languages than there are in English, so I would like to decrease as much as it's possible the number of vowels I have to practice, while learning more or less the standard accent.

The letter "a" is pronounced <em>differently</em> in "father" than in "mall" and "talk."  The "ou" in "thought" is similar to the "a" in "mall" and "talk." 

Please take a look at thefreedictionary.com      Look for the words you have mentioned.  Click on the icons of the loudspeakers, and on the icons of the American flag.  You will hear the words pronounced.

Accents do vary in different regions of the country.