Why a second Vandersteen sub?

Two subs load the room better. This creates a more seemless integration with the Sigs. It also allows for ultimately more total bass power(plays louder.) Lastly, some music has channel separation down into the lower frequencies. Two subs allow you to take advantage of this.
I have found that using two increases the soundstage width and and sounds more natural. It makes the system sound bigger with better dynamics.
I use the 3 Sigs with a pair of subs. It just doesn't sound the same with one.

Bigtee is correct. Also, make sure you are using the fixed filters. Several folks here make them, using better caps than RV, for lower cost. But at least, go the X-2s.

If you are using it with a 2-channel system it will improve
on the location and imaging. In my Linn system (2-channel)
I first used one AV-5150, ran L & R cables to it so that the internal circuit will blend both channels. Even at the
lowest setting on the sub's crossover (40 hz) you could feel the bass moving towards the side of the room where the sub was placed. In my listening room placing the sub between the L & R speakers was not an option. I got another
AV-5150, place each one in the rear corners and love the way they sound. You will find an excellent explanation on why two subs are needed in a 2-channel system in Robert Harley book, "The Complete guide to High-End audio".
Only one sub is used in Home Theater systems because the .1 channel of the 5.1 channels is a dedicated low effect mono channel. On some HT systems two subs or more are used to compensate for the room size. Hope this will hepl. Hector