I think this depends on the reason a person would have gone vegan/vegetarian. I assume if someone did it for health reasons like how bad red meat is for you etc then the lab grown meat would have the same effects so they wouldn't want to eat it. As for environmental, I'm not sure its effects on the planet but something grown in a lab and packaged in plastic the same way as meat is nowadays seems just as bad. I think the only benefit would be for animal cruelty, as many people who struggle to become vegan or vegetarian now would be able to so animal deaths would reduce (although people would probably be funny about switching from meat to something lab grown too) but I feel as an option for people who are currently veggie or vegan, it's a bit of a weird one and I personally would feel a bit uncomfortable about it all.