Phage existence as they multiply during productive infections.
Key is that a vegetative phage contrasts with a prophage. The concept of a vegetative phage, though, also contrasts with free phages and, perhaps, also with mature virions that have not yet been released from infected bacteria.
Another consideration is the degree of discreteness of vegetative phages, i.e., is the vegetative phage equivalent to an infective center, that is, other than in terms of the free phage aspect of infective centers? Or is an individual bacterial infection in some manner divisible into more than one vegetative phage?
Vegetative phage is (or was) approximately equivalent in its usage to the contrast between, for example, the bacterial vegetative state (or growth state, i.e., vegetative cell) versus a non-replicative endospore form. This comparison, though, comes with the caveat that prophages still replicate, just not via phage productive cycles.
This is the definition from Adams (1959), p. 442: "The form in which phage (or phage genes) multiply during the lytic cycle of phage growth." Keep in mind that this definition was authored prior to the discovery of chronically releasing phages, hence the emphasis on "lytic" cycle rather than "productive" cycle, though with the caveat that almost nobody employs the term productive cycle versus the substantially more familiar lytic cycle.
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