Simple Shader: Pushing vertecies towards Camera

Hey Guys,

I want to add a Glow to an effect. To do that i want to draw a particle that is pushed just a little bit towards the Camera to draw over very near object. (E.g. that particle should not clip into woodpieces of a fire effect, when its billboarded).

So I started to write my own Particle Shader. First i thought you need the Offset Parameter, but it didnt helped at all. So my first question is, wether the Offset x , y Parameter in Shaders only effect verticies that lie nearly on the same z position in relation to the camera.

Then i changed the Vertex shader and it did start to get something. But when I now get very near to the particle with the z Offset it is some kind of distorted. Do I something wrong?

Here is my vertex shader:

     v2f vert (appdata_t v)
             {
                 v2f o;
                 float3 ObjSpaceCameraPos = mul(unity_WorldToObject , float4(_WorldSpaceCameraPos,1.0)).xyz;
                 float4 offset =  float4(normalize( ObjSpaceCameraPos - v.vertex) * _Offset,0.0);
                 v.vertex += offset;
                 o.vertex = mul(UNITY_MATRIX_MVP, v.vertex);
                 #ifdef SOFTPARTICLES_ON
                 o.projPos = ComputeScreenPos (o.vertex);
                 COMPUTE_EYEDEPTH(o.projPos.z);
                 #endif
                 o.color = v.color;
                 o.texcoord = TRANSFORM_TEX(v.texcoord,_MainTex);
                 UNITY_TRANSFER_FOG(o,o.vertex);
                 return o;
             }

Hope someone can help me. Thank you,

Julian