Spirulina.  Supplements.

My usual deal on vegitarian - Since it's easier to do low fat with
vegitarian always try that first and only switch to low carb if
you can't sustain it.  Then ask yourself why you're a vegitarian.
If you're veg for religious, political or environmental reasons
stick with it and do what low carb workarounds you can find.
If you've veg  because you think it's healthier then correct your
thinking - Low carbing is more healthy than either extreme of
all meat or no meat.  Meat's the main source of B12.