vegging is the process by which you get your plants ready to bud. if you think about it, when plants are outside and are in a veg state, they start (in the northern hemisphere) coming out of the ground in late march early april when the daylight is getting longer and longer. the light continues to increase in time and intensity as it moves through the summer into the fall. (if anything there is more light during the veg cycle than the bloom, but that is neither here nor there) as you get closer to fall the plant senses a change in daytime light and flowers so that it can be pollinated and produce offspring in the form of seeds. obviously we don't let them get pollinated, but you see my point.
you need to have a consistent amount of light throughout the grow cycle to optimize your bud output. the light requirements during veg are crucial to get a healthy plant that can move the nutrients up to the buds when that part of the grow comes. so basically, you would not get the same amount of bud because your plants would not enough light to veg properly and might be weak and stretchy or something.
i guess that all depends on how many plants you have in a room under 2 1000 watt hps lights. if it's five then you're fine. if it's 100 then you need to up your light. hope that helps.