you can just do a dot product in the vertex shader and use the result to multiply with your alpha in the pixel shader:

define an hlsl function like this:
float4 Dofresnel(float3 N,float3 P)
{
float3 D = (float3)vecViewPos-P;
return (1 * dot(N,normalize(D)));
}

somewhere in your vertex shader :
float3 N = normalize(mul(inNormal,matWorld));
float3 P = mul(inPos,matWorld);
Out. fresnel = Dofresnel(N,P);

and then in your pixel shader just:
half fres=saturate(In.fresnel*3);
color.a=color.a*fres;


obviously this isn't complete code.. but it gives you the idea. I use this code for a fresnel lighting effect, but the principle is the same.
dot(worldnormal, normalize(viewpos-worldposition))