Rapid vegetative growth is how I’d define when veg starts. Seedlings are developing roots right off the bat more then anything as the ratio of roots to shoots is a key factor of health. Once the roots are established and the plant has a ready supply of resources it shifts to growing leaves to capture sun and produce sugar. That’s the start of vegetative growth as the plants need lees P (involved in cell division and replication, so needed for rapid root or flower/seed growth) and more N (which is used for all things green and leafy).
Hope that answers your question, their is no definitive answers so people suggest things like the number of leaflets or the number of nodes.