I agree with Ruthi, The best way to learn business English is to be around it as much as possible. I got lucky and have been living in the USA since I was three. Most of what I learned was from movies and songs. Business English was something that came natural once I began working in the USA. What are your biggest strengths and weaknesses in English?

The best way to improve your vocabulary is to expose yourself to as much English as you can.You should try to do something every day,be it reading,listening to songs,watching movies or talking to others in English.In this way you can enrich your vocabulary.