I have not heard the C5 with the Vsonic, but someone else probably has this combo and can comment.
Getting the music from the iPod to the C5 can be accomplished in two ways. You can use a 3.5 mm >3.5 mm cable. This takes amplified sound from the iPod headphone out jack, and feeds into the input on the C5 where it is amplified again. In this arrangement the volume control on the iPod will still be functional, as will the volume control on the amp. The preferred way, and the way to get the full advantage of the generally superior amplifier in the C5 compared to the amplifier in the iPod, is to use a line out dock cable which is a 30 pin (for docking connector on iPod) > 3.5 mm. In this arrangement, you will bypass the amplifier on the iPod (the volume control on the iPod will no longer function), and be feeding an unamplified analog signal directly to the C5, taking full advantage of the SQ of the C5. A photo of my rig is below. If you are going to go the route of an external amplifier, the small investment (about $10 on Amazon) for the Fiio L9 cable is necessary, in my opinion. (Just type Fiio L9 in the Amazon search box.) If you decide to go with the C5, I believe JDS Labs sells the Fiio L9 on their site.
Some folks claim that the custom LOD cables improve SQ. I am not one of them. The Fiio LOD cable is well made and inexpensive...does the job. I have had one for almost two years with out ever having any problems. The custom LOD cables can be in the $100-$200 price range. If I had that kind of $$$ available, my choice would be to upgrade headphones/IEMs.
Hope this helps.