What is a VN01A Transistor?
First off -- the schematic is using the wrong symbol.  The VN01A appears to be an enhancement mode part from its role in the circuit and the part number -- all Supertex depletion mode parts have a D in their part number such as LND01 or DN3765.  (Microchip still makes them, BTW.)

As to the part number itself, the VN01A is an older Supertex N-channel part that appears to be obsolete.  Given the noncritical small-signal switching application the circuit from the linked application note uses them in, and the note about the schematic symbol listed above, I'd substitute any jellybean N-channel FET in TO-92 or SOT-23.