It helps if we know what soundcard you're using.
Most, if not all, consumer soundcards have had their analog I/O color coded in the last 5 years. For most of us, the most important colors are: Lime Green - Line Out and Light Blue - Line In. The Mic In is usually pink in color. Note that with some soundcards and onboard audio, changing from ordinary stereo operation to surround mode (5.1, 7.1...) will cause some inputs to switch into output mode, often sacrificing the line and mic inputs.
Use the Vamp Pro's unbalanced 1/4" outs and use a good Y-cable; preferably, make one yourself if you're handy with a soldering iron. I've seen some y-cables and y-adapters that sum to mono. Remember that the tip in a stereo plug, be it 1/4" or 1/8", is always the left channel.