I don't see your actual vertex coordinates, but if it draws correctly as a point list, then it's probably a culling issue. You don't say how you turned off culling before, but try inserting the following into your draw function, to make sure that there is no culling just before the draw call:

bool ST_Prim_Triangle::Draw()
{
	targetWindow->GetWindowDevice()->SetRenderState(D3DRS_CULLMODE, D3DCULL_NONE);		   // this line
	targetWindow->GetWindowDevice()->SetStreamSource(0, vertex_object, 0, sizeof(D3DVERTEX));
	targetWindow->GetWindowDevice()->SetFVF(D3DFVF_XYZRHW|D3DFVF_DIFFUSE);
	targetWindow->GetWindowDevice()->DrawPrimitive(D3DPT_TRIANGLELIST, 0, 1);

	return true;
}

If that doesn't help, in PIX go to the end of a frame, select the color buffer window, right click on a pixel that is within the area the triangle should be drawn, and click "Debug Pixel". That should show each time the pixel shader was run, and if it failed the depth test, stencil test, etc.

I don't see your actual vertex coordinates, but if it draws correctly as a point list, then it's probably a culling issue. You don't say how you turned off culling before, but try inserting the following into your draw function, to make sure that there is no culling just before the draw call:

bool ST_Prim_Triangle::Draw()
{
	targetWindow->GetWindowDevice()->SetRenderState(D3DRS_CULLMODE, D3DCULL_NONE);		   // this line
	targetWindow->GetWindowDevice()->SetStreamSource(0, vertex_object, 0, sizeof(D3DVERTEX));
	targetWindow->GetWindowDevice()->SetFVF(D3DFVF_XYZRHW|D3DFVF_DIFFUSE);
	targetWindow->GetWindowDevice()->DrawPrimitive(D3DPT_TRIANGLELIST, 0, 1);

	return true;
}

I don't see your actual vertex coordinates, but if it draws correctly as a point list, then it's probably a culling issue. Try inserting the following into your draw function:

bool ST_Prim_Triangle::Draw()
{
    targetWindow->GetWindowDevice()->SetRenderState(D3DRS_CULLMODE, D3DCULL_NONE);		   // this line
    targetWindow->GetWindowDevice()->SetStreamSource(0, vertex_object, 0, sizeof(D3DVERTEX));
    targetWindow->GetWindowDevice()->SetFVF(D3DFVF_XYZRHW|D3DFVF_DIFFUSE);
    targetWindow->GetWindowDevice()->DrawPrimitive(D3DPT_TRIANGLELIST, 0, 1);

    return true;
}

If that works that means your vertices are in the wrong order. Either re-order the vertices or switch the culling mode during initialization.