I am a vegetarian and it didn't go against me. However I wasn't planning to make them veggie if they absolutely refused. I might not agree with eating meat personally, but considering I was planning both times to adopt older children (I adopted an 10 year old first time, second time an 8 year old), you can't really force a child of that age to go veggie - if they don't want to, they'll find a way around it!
I think it will depend on the individual social workers involved. If you are prepared to be very flexible and offer a child both meat and dairy at first, and transition them slowly across, then that would satisfy me if i were the sw! Also, what would you do if the child just point blank refused to stop eating meat? To the point of long angry tantrums lasting several hours? I think its important for you to know food is often a big issues with adopted children. Since they often were starved or withheld food, they can react very badly if they think they are being denied food. e.g. when DD2 moved in, if she asked for food, and I said 'no' she would go wild and cry and scream...in blind panic because she instinctively felt she was going to be starved. And once somone has been starved of food, that gets hardwired into the brain, and food issues are not easy to overcome..in fact they can last all of someones life
Of course not all children do have food issues, but I think you would need to show that you can be pretty flexible with a child like I described above, or indeed one who has been raised on finger foods and meat and nothing else (!) and is very very picky
Also, if i were the sw, I would ask about your DD - if she sees you offering a meat choice to her new brother/sister and she demands a taste...then how would you handle that?
But some people do see a vegan diet as deficient. If you get approved, you may unfortunately find you enquire about adopting a particular child, that childs sw rejects you because you are vegan (you will have a sw, the children have their own sw, so you need both onside). Ao yes, depends on your local authority and the individual sw's
Best of luck :)