Juan,
I am not exactly sure whether you are asking how to program the PLC to use your sent data or how to make the PLC continually send a "real time" reading back to your PC, so let me try to answer both.
To make the PLC initiate a stop at the desired position you must:
1) do a write statement in you VB program to a data register in the PLC.
2) do any necessary conversions to the data so that you can do a direct comparision of your target value and the input from the encoder.
3) write a comparision statement in the PLC to the effect: If Encoder Value >= Target Value then Set Stop.
Now, if you want to continually display the "real time" current position of the encoder, you will probably want to program your VB application to snag the Encoder Value from the PLC on a regular interval (every 1/2 second, for instance). I doubt you will want to make this a really short time span since this could create a bit of a burden on your PLC and the process itself.
Hope this helps.
Steve