Quoting the VBO Development guide, you can use the
aggregate option to get a list of all chosen entities.
aggregate (optional): boolean. If TRUE, all selected objects' IDs
(oids) will be packed in an array and passed as the first argument to
the action function, instead of passing each object one by one. This
is useful to implement aggregate functions that perform a single
operation on the whole selection set.
Note this guide is only for D6. I have created a D7 version but could not add an aggregate example yet.
However, in your hook_action_info() implementation, set
aggregate to TRUE (defaults to FALSE) and VBO will send a list of entity IDs (for you to perform additional related tasks).