Will a vitamin b12 deficiency lead to low hemoglobin?

Yes: It will lead to a macrocytic (large blood cell) hypochromic (low hemoglobin) type of anemia.

Absolutely: B12 is required to make the "heme" part of hemoglobin. If you do not have enough b12, you will not be able to make heme, and will therefore have a low hemoglobin level.

Yes: But don't assume your supplies of B12 or your overall nutritional status is adequate if you are not anemic. The neurologic damage in B12 / folate (folic acid) deficiency begins before the anemia.