Assuming those function keys can enable or disable external monitor use. There are still a lot of variables to why this isn't working, your function keys may be fine. Be more specific as to what led you to believe the function keys are not working.
Some questions:
Did the external monitor work before?
What made you think the function keys are the issue?
Are you getting no signal message on the monitor?
Is there and error message saying vga not recognized (doubtful)?
When you go into display settings does the second monitor show up?
Have you tested the external monitor on other computer (I know you said you tested vga cable)?
Considering how you answer these question, can lead to a more accurate and short answer.