Convert pg/mL to/from pmol/L?
Your question is how to do the conversion of the units of mole/liter to gram/liter. 
Mole/liter, written as mol/L, is an amount (in units of mole) per liter.

To do this conversion, we need to know the convert mole to gram or vice versa.

To this end, we need to use the physical property of molar mass in units of gram/mole. This is the mass per mole of substance, in your case the substance is Vitamin B-12. 
According to Wikipedia, the molar mass of Vitamin B-12 is 1355.37 g/mol.

$$\frac{200\times 10^{-12} \frac{g}{mL}}{1355.37 \frac{g}{mol}}=0.146\times10^{-12}\frac{mol}{mL}=146\times10^{-12}\frac{mol}{L}=146\frac{pmol}{L}$$