The monthly and fiscal year to date ridership numbers can be found in the Monthly reports which are available going back 1 year on the Amtrak Reports and Documents webpage. The Vermonter ridership, IIRC, are only for passengers traveling to or from stops north of Springfield. Trips on the Vermonter that are confined to the NEC are allocated to the NEC ridership stats and I believe trips to/from stops on the New Haven-Springfield are allocated to that corridor category.
Ridership stats by year with more details are available on the NARP stats webpage under the By Route link (which links to a single PDF file with 1 page for each service)