I don't recall the ghost ever stating surprise at the existence of the vault of glass. But even so, it should be noted that the guardians which went in there before went in there so long ago that their existence was questioned. Hence why Ikora stated "He is real..." after that one guardian stated his name, as if satisfied by the confirmation of a legend. Also, Venus is full of ancient tech of course, but there's no telling how much of a hot-spot it's been for guardian patrols in the years just prior to Destiny 1. It should also be noted that your ghost was already aware of what a Vex was and is. So his surprise was only from their presence on Venus and since it's shown he does have information on your enemies (including where their bases are in our system, as shown from our ghost knowing that the hive came from our moon) it's likely that either Venus wasn't patrolled much by guardians lately, or the Vex were tucked away and hidden for quite some time, maybe leaving their architecture behind with no explanation from them. Also it's likely the leaks are legit, considering an actual Megabloks set had a description describing it as Shaxx standing off against Cabal "red guard" in the City. This is an actual, real set from their website and not some fake "I made something in Photoshop then took a photo with my phone" leak, so it's likely real and it also likely suggests the Cabal are invading the city. And again, we were completely neutral with the Cabal until we decided to jump in and shoot shit on Mars in Destiny 1, so the Cabal couldn't be the ones invading if it were a sequel.

Dude I'm out that's too much thinking for this guy.