There are many of notes in the bass line of Variation 5.
(note that the right hand performs the assumed upper voices and we are not looking at the counterpoint in the upper parts Vivaldi wrote at this point).
Let’s see if we can understand how this diminution relates to the underlying voice leading pattern.
Each group of four notes has three notes from the underlying harmony and one note outside of it. That note leads to the next one in a goal oriented fashion. So this is simply an arpeggiation with an embedded goal oriented diminution.
Notice that almost all measures employ the exact same pattern of diminution. This is important since it creates a sense of cohesiveness. Once again we can define this prinicipal as the preference for diminutions that have occurred once they occur.