I found the problem, you need to set the input layout...

InputElement[] inputElements = new []
		{
			new InputElement("POSITION", 0, Format.R32G32B32_Float,0,0),
			new InputElement("TEXCOORD", 0, Format.R32G32_Float, InputElement.AppendAligned, 0)
		};
InputLayout layout = new InputLayout(m_d3d11_Device, m_vShaderSignature, inputElements);
m_d3d11_Device.ImmediateContext.InputAssembler.InputLayout = layout;
// Set the vertex buffer for the text geometry (a square).
m_d3d11_Device.ImmediateContext.InputAssembler.SetVertexBuffers(0, new SlimDX.Direct3D11.VertexBufferBinding(m_d2dVertexBuffer, 20, 0));

I found the problem, you need to set the input layout...

InputElement[] inputElements = new []
		{
			new InputElement("POSITION", 0, Format.R32G32B32_Float,0,0),
			new InputElement("TEXCOORD", 0, Format.R32G32_Float, InputElement.AppendAligned, 0)
		};
InputLayout layout = new InputLayout(m_d3d11_Device, m_vShaderSignature, inputElements);
m_d3d11_Device.ImmediateContext.InputAssembler.InputLayout = layout;
// Set the vertex buffer for the text geometry (a square).
m_d3d11_Device.ImmediateContext.InputAssembler.SetVertexBuffers(0, new SlimDX.Direct3D11.VertexBufferBinding(m_d2dVertexBuffer, 20, 0));

Also, I've adjusted your shader code to make things match up ...

struct VS_INPUT
{
float4 Pos : POSITION;
float2 TexCoord : TEXCOORD0;
};

...

VS_OUTPUT Text_VS(VS_INPUT input)
{
  VS_OUTPUT output = (VS_OUTPUT)0;
  output.Pos = input.Pos;
...

I found the problem, you need to set the input layout...

InputElement[] inputElements = new []
		{
			new InputElement("POSITION", 0, Format.R32G32B32_Float,0,0),
			new InputElement("TEXCOORD", 0, Format.R32G32_Float, InputElement.AppendAligned, 0)
		};
InputLayout layout = new InputLayout(m_d3d11_Device, m_vShaderSignature, inputElements);
m_d3d11_Device.ImmediateContext.InputAssembler.InputLayout = layout;
// Set the vertex buffer for the text geometry (a square).
m_d3d11_Device.ImmediateContext.InputAssembler.SetVertexBuffers(0, new SlimDX.Direct3D11.VertexBufferBinding(m_d2dVertexBuffer, 20, 0));

Also, I've adjusted your shader code to make things match up ...

struct VS_INPUT
{
float3 Pos : POSITION;
float2 TexCoord : TEXCOORD0;
};

...

VS_OUTPUT Text_VS(VS_INPUT input)
{
  VS_OUTPUT output = (VS_OUTPUT)0;
  output.Pos = float4(input.Pos, 1);
...

So here, it's ok to have a float3 for your position in the vertex shader input, as long as it's converted to float4 for the pixel shader.

I found the problem, you need to set the input layout...

InputElement[] inputElements = new []
		{
		    new InputElement("POSITION", 0, Format.R32G32B32_Float,0,0),
			new InputElement("TEXCOORD", 0, Format.R32G32_Float, InputElement.AppendAligned, 0)
		};
InputLayout layout = new InputLayout(m_d3d11_Device, m_vShaderSignature, inputElements);
m_d3d11_Device.ImmediateContext.InputAssembler.InputLayout = layout;
// Set the vertex buffer for the text geometry (a square).
m_d3d11_Device.ImmediateContext.InputAssembler.SetVertexBuffers(0, new SlimDX.Direct3D11.VertexBufferBinding(m_d2dVertexBuffer, 20, 0));

Also, I've adjusted your shader code to make things match up ...

struct VS_INPUT
{
float3 Pos : POSITION;
float2 TexCoord : TEXCOORD0;
};

...

VS_OUTPUT Text_VS(VS_INPUT input)
{
  VS_OUTPUT output = (VS_OUTPUT)0;
  output.Pos = float4(input.Pos, 1);
...

I found the problem, you need to set the input layout...

InputElement[] inputElements = new []
	    {
			 new InputElement("POSITION", 0, Format.R32G32B32_Float,0,0),
		    new InputElement("TEXCOORD", 0, Format.R32G32_Float, InputElement.AppendAligned, 0)
	    };
InputLayout layout = new InputLayout(m_d3d11_Device, m_vShaderSignature, inputElements);
m_d3d11_Device.ImmediateContext.InputAssembler.InputLayout = layout;
// Set the vertex buffer for the text geometry (a square).
m_d3d11_Device.ImmediateContext.InputAssembler.SetVertexBuffers(0, new SlimDX.Direct3D11.VertexBufferBinding(m_d2dVertexBuffer, 20, 0));

I found the problem, you need to set the input layout...

InputElement[] inputElements = new []
			    {
				    new InputElement("POSITION", 0, Format.R32G32B32_Float,0,0),
				    new InputElement("TEXCOORD", 0, Format.R32G32_Float, InputElement.AppendAligned, 0)
			    };
		    InputLayout layout = new InputLayout(m_d3d11_Device, m_vShaderSignature, inputElements);
		    m_d3d11_Device.ImmediateContext.InputAssembler.InputLayout = layout;
		    // Set the vertex buffer for the text geometry (a square).
		    m_d3d11_Device.ImmediateContext.InputAssembler.SetVertexBuffers(0, new SlimDX.Direct3D11.VertexBufferBinding(m_d2dVertexBuffer, 20, 0));