First of all , how can one prove that gases have no fixed volume ? And if (though i know that it have) lets take a case . Take small amount of gas and fill it into a large box , about size of earth , consider that box has vacuum in it . The gas should fill all the earth but if does that , then the intermolecular attraction force should be inconsiderable or negligible at such large distances . What happen then ? There should be a limit to the volume ...
Another question is why molecule let H2 not attract another gas particle let He as there is same van der Waals force ........