You can send emails to all your members if you sign all usergroups up as Default Subscribed
Note that vBMail will not override the "Receive Emails from Administrators" checkbox - users who unticked that will be skipped regardless of their subscription settings. This is to comply with spam policies set forth by email providers like Yahoo, Hotmail, Google Mail, etc.
Hosts may impose a "maximum emails per hour" restriction yes. You should contact them to figure out if they impose such a restriction if you find yourself unable to email all your members.