Green Lanterns can also do intangibility, and of any type they want. Diamond hardness is also not much of a defense against something with the raw power of a GL ring. Any rookie GL would be able to perform any feat Vision can do and much more besides. If Vision tries to go intangible then Hal just causes his attack to match it. The yellow on Vision's costume is not much of a defense considering how much of his surface is not yellow. In addition, Hal could cause his attacks to leave the dimension they are in and reenter the dimension inside vision.