Can I directly connect Vocm of all eight ADA4932 to the Vcm output of the AD9257 or do I need to buffer Vcm?
Yes, you can connect the AD9257's Vcm pin to 8 ADA4932s. As the ADA4932's VOCM input impedance is 25kOhms, driving 8 such loads in parallel would be no problem and 8 of these in parallel would be 3.125Kohms.
With the AD9257’s Vcm of 0.9V, the Vcm would have to drive 0.9/3.125K = 288uA.