Is there a correlation between B12 deficiency and cardiovascular disease?
Is it known whether low B12 causes cardiovascular diseases?


Yes, it is known. B12 deficiency does not directly cause cardiovascular disease, and I would be suspicious of any health site that says it does. B12 deficiency causes hematologic and neurologic problems. 


  Why is low B12 correlated with high homocysteine:


Low B12 is correlated with high homocysteine because it is a cofactor for the conversion of homocysteine to methionine by homocysteine methyl transferase. Low folate will do the same thing (cause an elevation in homocysteine). 



These cofactors and metabolites are involved in methyl donor reactions that are particularly important in the production of blood cells and myelin. Strict nutritional B12 deficiency (reduced intake) often occurs along with other nutritional deficiencies.

The primary hematologic problem is anemia, with associated changes in the morphology of certain white blood cells. The anemia, if severe and prolonged can contribute to cardiac problems of other causes. You also see paresthesias (often symmetric), a shuffling gait, and other odd neurological problems including depression and psychosis.  B12 supplementation does not always reverse the neurological problems.

You can read about B12 deficiencies in Cecil medicine Chapter 225 and 167, Harrison's Chapter 74, and in this review.