Thanks @TwistedTwigleg for your welcome and for the hint.
I've managed to find the solution and get the expected result.
I made two mistake in the previous code. The first one is that I needed to inverse the Y channel of my UV channel.
The second is more tricky, and i've spend some time before understanding that what is commonly called view matrix is called INV_CAMERA_MATRIX in Godot.
So, to get each vertex facing toward camera, you simply need to multiply your centered UV by the view matrix and add the result to the vertex position.
Here is the code for anyone is interested :
shader_type spatial ;
uniform float offset_multiply = 1f;
void vertex() {
vec2 centered_uv= UV;
centered_uv= (UV *2f -1f);
centered_uv.y *=-1f;
vec4 offset_UV = vec4(centered_uv,0,0) * INV_CAMERA_MATRIX;
VERTEX += offset_UV.xyz *offset_multiply;
COLOR = vec4(UV,0,1);
}
void fragment() {
ALBEDO = COLOR.xyz;
}
Thanks again,