When you create a list in VerticalResponse from within Salesforce you will receive an email summary Load Report of the records loaded and not loaded into the list. Previously bounced and unsubscribed records are scrubbed from the list at the time of import. Any discrepancy between the original size of a list of leads and/or contacts in Salesforce and the final list in VerticalResponse is most likely due to removal of bounces and unsubscribes you received from previous emails.
Results from the import are shown in the email summary Load Report. If you missed the summary load report email the first time you can test by importing your list again.