This is going to be tricky. Normally I recommend to get it working in Vegas without TandemVids and then use the same settings in TandemVids but you've done that. It sounds like you've done a lot of trouble shooting. I haven't seen the issue before and I can't replicate it here. Your computer should be more than capable of rendering faster than real time.
All I do is ask Vegas to render and then wait for it to finish. The code is fairly simple so there isn't too much that can go wrong at the TandemVids end.
I have a couple of suggestions but I'm not really sure they'll help.
Try a different rendering engine and template just to see if it's specific to that particular template. I have seen issues with custom rendering templates created in earlier versions of Sony Vegas and then selected for use in TandemVids and Vegas 11. The issue is different because when you render manually using the custom template, Sony Vegas throws an error, which is what confuses TandemVids. It hangs and doesn't start rendering at all.
I've had one person recently who was experiencing long render times - not six hours - who disabled resampling on the empty events to fix it.
Try setting the render thread count to 1. Close TandemVids, open a separate copy of Vegas to change settings and then restart TandemVids.
Try disabling the GPU for rendering. Close TandemVids, open a separate copy of Vegas to change settings and then restart TandemVids.
Try updating your video drivers, especially if the previous option resolves it.
Mick Hardy attached the following image(s):