Yes, your manifold vacuum is decreasing. Don't worry, it is normal.

Your "Economy" gauge measures manifold vacuum, which is a simple indicator of engine power production. Gasoline engines have a high vacuum at Idle (throttled, throttle plate closed) and a low vacuum at Full Open (unthrottled, throttle plates open.)

When you accelerate, you need power, so you open the throttle and the vacuum goes lower.

At a steady cruise you should see higher vacuum, or more 'economy' than when accelerating, and when you lift your foot to slow down it should read really 'Efficient.'

So, according to your gauge, your best 'Economy' is idling or decelerating, but you won't go very fast...