If you have several vESA in cluster - the choice is really yours, depending on your mail flow and setup. Having the vSMA will help w/ centralized management of all quarantines, reporting, and message tracking. Once the 9.x vSMA is available, I am sure we will see w huge up-tick in customers using the vSMA in this fashion.
As for the license vs. service bundle - you'll need to engage a conversation directly with sales/account team, or your reseller. If you have the proper bundle purchased, you only need one valid VLN (virtual license number) that is associated to the appliance --- and you can use the same license across multiple virtual installs. You will however need a separate purchase and license for the vSMA - as that will have it's own VLN tied to the license and appliance.
If you have hardware appliances already - you can automatically get a 1 year virtual license for your vESA, vSMA by requesting for a matching virtual license for your production ESA serial number.
Go to: https://tools.cisco.com/SWIFT/LicensingUI/Quickstart and click Get Other Licenses > Email and Web Security > Get Activation Codes
For product, choose SW Bundles. Enter in your hardware serial number, only has to be the trailing digits and numbers after the "-". Click Request Code and continue through the rest of the process.
For more help, further knowledge:
http://www.cisco.com/c/en/us/support/docs/security/email-security-virtual-appliance/118301-technote-esa-00.html
-Robert
