The Vegetative Stage is one of the two main major stages for your tomato plant as different from the other stage, flowering.
Your plant is going to remain in the vegetative stage as long as you are giving it between 18 to 24 hours of lighting a day. The growth cycle here is only 2 to 4 weeks long for most of the indoor varieties and this completes in the third to fifth month of growth depending on the indoor grow conditions.
What happens during this stage? What is the plant doing? The plant is focusing its energies on developing its leaves and root system fully. This is all in preparation for its flowering phase.
What type of things will you observe with your plants during Vegetative Stage? Hopefully you will be noticing more leaf production as your plant grows. Greater numbers of larger leaves help insure faster growth, and with optimal hydroponic conditions you could see up to six inches per day! You will know you have passed the halfway point of vegetative stage when the number of blades per leaf starts to decrease as does the distance between the sets of leaves on the stem.