Although it is never explicitly stated in the book, in this SciFiNow interview Gaiman confirms that Silas is indeed a vampire.
Burial clothes usually have open backs that can be fastened. This is because it is very hard to manoeuvre a dead body into normal clothes.
The creatures wearing these suits must have acquired them from buried corpses.
The name of the ruined city of ghouls is derived from the German word "heim", which means "home".
Gaiman's descriptions of the horizon's red sun and the nightmarish angles of Ghûlheim's battlements summon imagery from another of his projects, The Sandman: Season of Mists.