This step is very easy: you only need to download and install the VAC software. Once it is successfully installed, there is no need to even open VAC or configure VAC -- VAC runs silently in the background, automatically creating a music stream called "Line 1 - Virtual Audio Cable". We will use this Line 1 in an upcoming step.
Trial version of VAC available here
Full version of VAC available here
Other versions of VAC are available at various download sites around the Web.
VAC is 'routing' software for audio. This means: VAC lets you transfer music and voice signals from different software packages and microphones to play in other software or speakers/headphones of your choosing. This obscure-but-useful tool is the key to streaming music while also maintaining full voice communications in ventrilo.
VAC is a product authored by Eugene Muzychenko, a gifted programmer.
More details on this VAC requirement are available here