Unfortunately you cannot select all mask keyframes in the timeline and then move only one vertex and have it effect all vertices. You can select all keyframes for position for example, then click on a position keyframe in the Comp window and drag the entire group around.
Nice feature request though.
Thanks for clarifying,..
your right, that is truly unfortunate.
Guess that just leaves "KeyTweak" as the exclusive answer particularly for path Verticies. There are plenty of scripts to uber edit layers and or layer properties, but only one that works with arguably the most used facility in AE, ie: Masks !!
Thank God for Mathius at Mamoworld, the most unsung hero of After Effects.
PS: feature request is an appropriate suggestion, but my request and just simply selecting ALL keyframes without very slow and endless drag-scrolling around 1000's of keys, both have already been requested for years, probably a decade or two.