Controlling the frequency of oscillation of SI530 VCXO
What gave you the idea that Si530 is a VCXO? It says in the datasheet: 


  The Si530/531 IC based XO is factory configurable for a wide variety
  of user specifications including frequency, supply voltage, output
  format, and temperature stability. Specific configurations are factory
  programmed at time of shipment, thereby eliminating long lead times
  associated with custom oscillators.


Maybe you should be looking at this page: http://www.silabs.com/VCXO