Problem with removeAllRanges ()

I'm developing a javascript mask for an input and am having a problem with the selection.removeAllRanges(); function every time I use it, it removes the cursor from the input and I can not use the backspace key to delete the values contained in input.

function fdecimal(vobj,vint,vdec)

{     var selection = window.getSelection ();     selection.removeAllRanges ();

document.getElementById(vobj.id).select();

vlista = "-0123456789";

vchar  = String.fromCharCode(event.which);

/*=== Caracter digitado deve estar parametrizado na lista (vlista) ===*/
if (vlista.indexOf(vchar,0) == -1)
    return event.keyCode = 0;
vaux = vobj.value + vchar;
vaux = vaux.replace(",","");
/*=== tratamento sinal negativo ===*/
vneg = vaux.indexOf("-",0);
while (vaux.indexOf("-") != -1)
{
    vaux = vaux.replace("-","");
}
/*=== Tamanho máximo numero ===*/
if(vaux.length > (vint + vdec)) {
    return event.keyCode = 0;
}
/*=== Completa zeros a frente ===*/
vaux = "" + Number(vaux);
for (i=vaux.length;i<=vdec;i++)
{
   vaux = '0' + vaux;
}
/*=== Coloca virgula como separador de decimal ===*/
vret = vaux.substr(0,vaux.length-vdec) + ',' + vaux.substr(vaux.length-vdec,vdec);
vobj.value = (vneg!=-1?'-':'') + vret;
selection.removeAllRanges();    

}