Hi Ales,
Any backup (snapshot or traditional) needs to contain a complete set of related data to be useful for restore. So to properly back up your database application (Exchange, SQL, Oracle, etc.), you should always use some sort of agent to bring the database in a consistent state. Meaning that any data in the RAM is written to disk, the logfiles are up to date and the database is quiesced.
You use SME or SMSQL to bring your database in that consistent state. SMVI doesn't bring your application in a consistent state, only the OS (if you incorporate VMware snapshots). So SMVI is great for static data, such as application binaries, OS and webservers (without database), but not for databases. On the other hand, SME and SMSQL don't backup the OS and application binaries.
1 + 2) So in short: you'll need both solutions. 🙂
3) They'll work perfectly together, but you might want to consider not scheduling snapshots with both solutions on the exact same time.
4) Nope. Think about it: you use SME/SMSQL to create consistent snapshots of your databases. You can't (storage)snapshot a single vmdk. You'll need RDM's for that.
5) Not sure here, but there really is hardly any difference with a physical environment.
Hope it helps.
Bjorn