What role does the Vatican play in marriage annulments? 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.