Rather than list all the studies here I'll just link you to a page with that lists them.

http://healthpsych.psy.vanderbilt.edu/2008/VeganDiets.htm

That page lists studies showing B12 deficiencies, bone density issues, and elevated Homocystein issues.

Also there is a very strong link to issues with pregnant and nursing women and their babies due to B12 deficiencies. The opinion of most researchers in this area appears to be that pregnant and nursing women should not follow a strict Vegan diet.