2007 Other Toyota Models Lowered fuel economy

I have a 07 Toyota Yaris that I purchased in the fall of 06. For the first year and a half I enjoyed excellent fuel economy getting roughly 300 miles per tank in the city. Then a month ago my fuel economy dropped by 100 miles per tank! My Toyota dealership serviced the car twice and could not give me an explanation or a solution for my sudden poor fuel economy! They told me all their "diagnostic tests" showed the car was fine and in proper working condition. I have noticed the car is more sluggish around 30 mph and that the accelerator vibrates like it never did before at 50 mph. So I guess my question is why would I be experiencing sudden poor fuel economy, what factors can contribute to this and what can I do to remedy the situation?
Thank you!