You may also want to see this StackOverflow topic about VistaDB.
Nothing is faster than SQL Server for what it does. If you think SQL Server is too slow you are probably doing something wrong. It is incredibly fast. There is no way a 100% managed engine is going to be faster than SQL Server on everything.
Lighter on resources - Yes
Easier to deploy - Yes (xcopy deployment)
Desktop database - Yes, you can use it for desktop development of applications. Some people also use it for multi user applications on a shared drive. Websites are another target where you can't install a service or don't want to pay extra for SQL Server access on the hosting account.
Security - VistaDB runs in-process with your application. There is no concept of user security since you are loading it directly.
Concurrency - Depends on your application. We support multiple processes access the same database, but some operations will still have locking issues you have to work with in your code.