The man Basch stabs mentions that he has a brother in Dalmesca if Im not mistaken which I think is where Vaan is living. When Basch stabs the soldier he mutters Vaans name. So I see a connection between them there. We also see Vaan brining flowers to the man in the white room so another connection although you see his body dissapear as from the chair as Vaan lays flowers on the chair in the white room suggesting his brother is dead and Vaans paying respects. The only reason I think Vaans brother and Ashes husband are the same person is really through resemblance and death but thats not much to go on.