The friend asks for the person to buy them a new vase, but the person refuses.
Well, intentional or not, the damage's already been done to the host's property and the guest cannot say that s/he's completely free from the responsibility. So even if the host did not request for payment, it should be the right thing for the guest to offer some payment to cover for the host's lost property. In terms of kamma, since the cause was unintentional, the fruit will certainly be of a different degree of severity than that of an intentional one. But that doesn't mean it'll guarantee that the guest will 100% be free in the future. Who knows, maybe in some near future, this guest might invite some of his own guests over, and his own guest in turns, will unintentionally break some property in his house! In short, if in doubt, try to play it safe and try one's best to be reasonable and responsible for one's own actions.