Significant advantages? No, not really if you plan on using VDJ.
Using the NI Soundcard, you might find you have a slightly reduced latency, but it probably wouldn’t be a huge amount.
The biggest advantages to come from having an Audio 6 or 10 is the bundled Traktor Scratch Pro licence, and the ability to use Timecode.
While in your set-up, Timecode is unneccesary as the CDJ 900’s work as USB controllers, if you want to play out at clubs/other friends houses and they don’t have a USB capable setup (For example, they have CDJ1000’s and a DJM800), you can bring your soundcard with you and plug in that way. Then you would use Timecode CD’s to play your music through traktor. Timecode also allows you to play on Turntables using special Vinyl discs, using them to control Traktor.
You may be lucky, and all the closest venues are rocking CDJ 900’s/2000’s, and DJM 900’s, so this wouldn’t be an issue at all. The biggest advantage to having a soundcard like above is flexibility… but it’s not neccesary in your case.
As for VDJ, there are plenty of people on this forum who use and swear by it. I use Traktor purely because it’s what I’m comfortable with, and works best for my Setup.
Of course theres one other option, being Serato Scratch Live, which is very stable and rock solid, however you can only use with particular Serato certified gear. (That is, a SL1/2/3/4, or particular Rane mixers). Serato is a fantastic alternative to Traktor and VDJ, but the costs of getting established is pretty steep.
Have a play around with VDJ, and do some research on Traktor and Serato. All three are fantastic pieces of software, and don’t let anybody tell you that one of them is the ‘be all and end all’. They’re all suited to different people and different workflows 