It's a myth that you need to have TONS of protein. Read the China Study. Too much protein is linked to cancer. Eat your legumes, Nuts, Chia seeds, etc. and you'll be fine. Otherwise there are vegan protein powders that will do the trick.
If you've only been a Veg for a few weeks and are tired it isn't from the B-12. B-12 is stored in the body and doesn't diminish in a snap. However, you will need a multi-vitamin or eat foods supplemented with vitamin B-12.
If you swap meat for too many sugars and white starches it will cause blood sugar fluctuations and you will have issues with energy too. When I don't watch it that is what happens to me, and why I gain weight.
Try making a green smoothie. Throw in some fruits, and top off by adding Romaine, baby Spinach or Bok Choy and you will have energy coming out of your ears. They turn me into a spaz!
Oh, also the fatigue might be your body detoxing off of the meat and junk. It can take a few weeks to flush all that out.