You do need VAC or VB Cable. Otherwise you'll create a loop (audio that goes out through sound card is processed and outputted again).
In fact, based on your description, it worked fine before (when you heard the "trial" voice): the audio was received by Stereo Tool and played to the sound card. But, you do need to make sure that all other programs send their audio to VAC instead of to the normal sound card.
The error when you removed VAC also makes sense: the sound card that you selected earlier suddenly didn't exist anymore.
If you want to run everything through VAC or VB Cable, set it as the default sound card. Then in Stereo Tool select the virtual cable as input, and the actual sound card as output. That's all.
No, that also didn't work. I've selected "Microsoft Soundmapper" in VAC as input and "Cable 1" as output. In Stereo Tool I've seleted "Cable 1" as Input and my sound card output as output.
But Stereo Tool just didn't do anything. There was nothing to see in the audio visualization panel at the bottom. No function changed anything. As if it's bypassed. I've checked it several times - no difference when active/bypassed. It just didn't process anything. I know how it should sound, because I'm using it sucessfully as WinAMP plugin.