I've heard that every annulment must go through the Vatican in some capacity. Does every annulment get reviewed by some tribunal there or is it just a rubber stamp for the tribunal of the diocese from whence it came?
My understanding has always been that this is handled by a Diocese's Marriage Tribunal.
On looking it up, it appears that when you request a Declaration of Nullity in Canada, an investigation and judgement are made by the Diocesan Marriage Tribunal, then all decisions are reviewed by a Canadian Appeal Tribunal, which issues the final Declaration (or not.)
This pamphlet (pdf) from the Toronto Archdiocese has a good explanation.