Donate, Legerdemain, Gauntlets of Chaos, and a lot of tricks using Animate Artifact can be used to give an Italian Mana Vault away. More importantly, of the six cards that Great Wall protects against (Traveler's Cloak, Giant Slug, Aysen Highway, Righteous Avengers, Illusionary Presence, and Zodiac Rooster), two are from Legends, one is from Portal Three Kingdoms, one is from Homelands, one is from Ice Age, and one is from Invasion. Anyone stupid enough to think that Great Wall is worth using most likely wouldn't have played long enough to have it or to have
heard of the five pre-Alliances cards. Moreover:
Traveler's Cloak is used almost exclusively in draft, or by players who, having recently graduated from Pokémon, wouldn't be playing against people who own enough cards to have Great Wall. (Of course, there are exceptions, but oh well.)
The same is true of Giant Slug, except that a) it's about 20 times as hard to get as Traveler's Cloak and b) that card might be used by someone who is obsessed with it (namely K9Archmage, for one), but most likely no one else.
Righteous Avengers has the same situation as Giant Slug, except that it is too boring for someone to be obsessed with it.
Zodiac Rooster is from Portal Three Kingdoms, which is illegal. I've never even seen one of those cards anyway.
Aysen Highway would provide as much advantage to you as it would to your opponent, because it is almost 100% guaranteed that an opponent controls a plains (the mana cost is 3WWW), and assuming that you have creatures (which you should, since
all Great Wall does is help you block), your creatures would be able to attack unblocked as well. Great Wall prevents you from having this advantage as well as it prevents your opponent.
Illusionary Presence is directly inferior to Phantom Warrior, which isn't that good anyway and has been reprinted in the basic set, while the Presence was only printed in Ice Age. Who would use a Phantom Warrior with cumulative upkeep that isn't unblockable if the opponent controls no basic lands instead of Phantom Warrior? After all, the Illusionary Presence was a rare in a set that hasn't been printed for three years. Phantom Warrior was an uncommon that appeared in Weatherlight, Portal, Portal Second Age, probably Portal Three Kingdoms, probably Starter, and definitely Sixth Edition. Someone who was searching for an
efficient nearly unblockable blue creature for three mana wouldn't find one (

), but would settle for Phantom Warrior or Metathran Elite, not Illusionary Presence.
Meanwhile, look at the best example of a card that can be used with the Italian Mana Vault for great effect - Donate. Can anyone remember this card? I can't. Oh yeah, it was the one that should be banned in Extended.
Once someone sees the Italian Mana Vault, they'll immediately think of the Donate-Mana Vault combo. Thus, it has exposure.
One of the two most important things: YOU DECIDE WHETHER OR NOT TO USE AN ITALIAN MANA VAULT. YOU DON'T CHOOSE WHETHER YOUR OPPONENT USES ZODIAC ROOSTER OR ONE OF THESE OTHER CARDS.
The other most important thing: Donate + Italian Mana Vault is an OFFENSIVE COMBO. It actively deals damage to your opponent. Meanwhile, Great Wall is a weak DEFENSIVE card that relies upon other cards to be of any use (remember, a global enchantment can't block when there is no Opalescence in play. You need creatures.). There is little sense in blocking when you can attack faster. However, if you
would pull back your White Knight to block your opponent's Righteous Avengers, except that you can't because you control plains, you need serious help.
BTW, aren't we talking about the English versions of the cards anyway? In that case, Great Wall is definitely worse.