Are you in The Hollows or Eden? Those are the only places you can not use the Serpent Holes since none of Vulgrim's shops are present. If that isn't the case, then I don't know what else to tell you besides the usual advice of clearing your cache and/or recovering your gamertag.
151/152 games completed (including DLC)
Completion percentage: 99.90%
Latest retail completion: Crysis
Latest arcade completion: Dust: An Elysian Tail