I am having a problem with reinstalling stopper on a vanity sink, It is an old sink that has a pivot rod centered in middle of drain pipe

First you must remove the old pop-up lavatory waste. Not always the easiest thing to do. Remove the "J" bend of the trap from the tail piece of the drain and the linkage clip from the plunger actuator on the drain body. Now loosen the spud retaining nut under the chrome drain. The chrome drain on top,(the part the plunger sets on) has threads. Try to unscrew the drain body. If the two parts have fused togather and are hard to remove you must hacksaw the two pieces between the nut and the bottom of the lavatory. Now we are ready to install the new pop-up waste. Clean the old putty from around the drain hole and roll out a small ring of plumbers putty. Form this ring around the chrome drain for a seal. Now screw the drain body into the chrome drain and tighten the spud retaining nut and washer. Next push the plunger actuator all the way down and attach the linkage to the actuator. Hand tighten the knured nut holding the plunger actuating rod. Replace the "J"bend,(now would be a good time to replace the old trap washers if your trap has them). Put the plunger in the chrome drain and your in business. And I bet you thought this would be a easy operation. Hope this helps and thank you for rating my reply. Tom