there are no a-d converters in the parallel port.
you'll need to build an A-D converter which you can then plug into your parallel port.
this could be done in 2 ways.
either a simple op-amp comparator this will however limit your range of voltage you can read to 8 steps.
or you could use a converter that you could buy.
or program your own converter using PIC chips.
or use a large op-amp circuit with a lot of logic circuits after it to make a proper 8 bit A-D converter this will give you 256 steps in your readings.
Personally however, I think that if you are going to do this then the best way is to buy or make a thing to read the voltage and transform the analogue signal (voltage) into a binary data signal.
you'll find most of these shop bought ones will use a serial link rather than a parallel link as parallel ports are getting rarer and rarer now!
also using the serial port in VB
is easier than using the parallel.