Try this for billboarding

[float3]

                //Check if we're in VR, if we are set the camera's position to between
                //the two eyes. This avoids bad stereo effects where the sprite is aiming
                //at each eye at the same time. Otherwise just use the normal camera pos.
                #if UNITY_SINGLE_PASS_STEREO
                    float4 cameraPos = float4((unity_StereoWorldSpaceCameraPos[0] + unity_StereoWorldSpaceCameraPos[1])*0.5, 1);
                #else
                    float4 cameraPos = float4(_WorldSpaceCameraPos,1);
                #endif
                
                float4 center = mul(unity_ObjectToWorld, float4(_rotPos.xyz, 1));
                cameraPos.xyz -= center.xyz;
                //cameraPos =  mul(unity_WorldToObject, cameraPos);
                float len = distance(float2(0,0) , cameraPos.xz);
                
                //Length of the hypotenuse of the triangle formed by the camera's
                //objectspace pos, the origin, and the projection of the camera on
                //to the xz plane
                float hyp = distance(float3(0, 0, 0), cameraPos.xyz);
                
                // rotate the vertices along the X-axis to face the Camera
                float cosa = len/hyp;
                float sina = (-cameraPos.y)/hyp;
                float4x4 R = float4x4(
                    1,    0,        0,        0,
                    0,  cosa,    -sina,    0,
                    0,    sina,    cosa,    0,
                    0,        0,    0,        1);
                pos = mul(R, pos);
                
                //Rotate the vertices around the y-axis
                cosa = (cameraPos.z)/len;
                sina = (cameraPos.x)/len;
                R = float4x4(
                    cosa,    0,    sina,    0,
                    0,        1,    0,        0,
                    -sina,    0,    cosa,    0,
                    0,        0,    0,        1);
                pos = mul(R, pos);