YAB a bit U, she's old enough to know her own mind & you really cannot force her, it will push her away from you & DH big time :(
However I can see that you are concerned for her health, or I would be saying YABVVVVU
No need to worry if you treat it as a nutritional learning exercise & let her know that you will only support her IF she eats a properly well balanced diet & this really isn't difficult without meat or animal products
Will she eat eggs for example ?? I know plenty "vegan" who will eat locally sourced proper free range eggs, but nothing else
If so a good nutritional breakfast is for example ...
1 egg, blitzed with 1 banana, with a tablespoon of sesame & sunflower seeds
Cook this batter as you would drop scones or pancakes & serve with black treacle or molasses - this is a nutritional powerhouse, with plenty of calcium & iron too -
if she won't eat "kind eggs" replace the egg with almond milk & add a bit of flour & oats too
Give her Spirulina (compressed pills, not powder)as a supplement, this is packed full of iron, (more than liver) B12 & much, much more
My daughter has grown up practically vegan as she is vegetarian, but had problems tolerating dairy & eggs too for a while, so she often rarely ate either. Despite this & long term ongoing stomach trouble meaning that a lot of what she ate flew straight through her, she has never tested as nutritionally deficient in anything