Sounds more (to me) like an intake restriction than a vacuum leak.

If you remove the big black hose from the throttle body, you can look in and see if the throttle plate moves as you tug on the throttle cable. If it's not moving freely, the engine won't be able to gulp enough air to rev as it should.

Also, see how it runs with that hose disconnected. If it revs okay like that, there may be a restriction "forward" of the throttle body -- blocked air filter, kinked hose, etc.