My car idles fine, but when I accelerate I have lost power and hear hissing sound. But when I get up to speed the sound disappears.

How would I know if the sound is being produced by a bad header or exhaust system or from a bad head gasket.

 

I think its a vacuum leak

 

Not a vacuum leak. There's no (or very little) vacuum when accelerating, therefore vacuum leaks have little or no effect at that time. A vacuum leak would hiss at idle, disappear during acceleration, and re-appear once cruise speed was reached.

Not a head gasket. When those go bad, you'd be posting about something REALLY SERIOUS; not just some weenie hissing sound.

More likely, catalytic converter stopped up; or, pieces of it have busted up, and are lodged in the muffler, and THAT'S stopped up instead.