Look at the example below. We have two different cultivars of Cleome grown next to each other in the flower trials, so they experienced the same growing conditions. The one on the left has weaker growth and less branching, while the plants on the right are full and healthy - you can't even see the ground in the middle of the plot! It is a clear choice that Cleome 'Senorita Rosalita' has better vigor and it has been a favorite on the Prairie Star list for years.
Here is another example from the K-State flower trials. First is a plot of Tecoma that definitely has good vigor. The plants are healthy, thick and have lots of flowers. However, the beautiful yellow flowers are mostly lost in that healthy green foliage and not as noticeable as we would like them to be. The flowers also point either straight out or down, further reducing their visibility.
Now compare that to the petunias below. These plants hold their flowers up over the foliage, so that the foliage is hardly visible and all we see is color. This is the sort of thing we look for when rating flowers to go on the Prairie Star list. Floriferousness = WOW !!! Look at all that color. Isn't that why we plant annual flowers?