You need to pass the modified user variable to MembershipUser.UpdateUser.
From the MSDN article on http://msdn.microsoft.com/en-us/library/system.web.security.membershipuser.lastlogindate.aspx:
You can also modify the last login date by setting the LastLoginDate
property of a MembershipUser and passing the modified MembershipUser
object to the UpdateUser method.
write this code
If Membership.ValidateUser(Login1.UserName, Login1.Password) Then
user.LastLoginDate = DateTime.UtcNow()
MembershipUser.UpdateUser(user)
e.Authenticated = True
End If
Cheers