If A's statement is true then it means that C's statement was fake, which means C did break it, but A's statement said that D broke it, and C and D can't both broke the vase, so A's statement is fake;
If B's statement is true then E broke the vase, but C's statment would be fake and make C the one that broke the vase, so B's statement is fake;
If C's statement is true then D's statement would be fake, so A was telling the truth when he said D broke it, but A's statement is fake, and so D couldn't have broke the vase, so C's statement is fake;
If D's statement is true then A's statement was fake, and that D didn't break the vase. This also means that B and E vouched each other, therefore they didn't break the vase. This statement doesn't cause any error, so C broke the vase. But just to be safe, let's also check E's statement.
E's statement is the same as B's statement, so E's statement is also fake.
Therefore, D's statement is true and C broke the vase.