In a certain context, the question will become much clearer, but it is usually used to refer to an ordinary object (or type) without "special authority". For example, if the serialization scheme claims to work with "vanilla objects", you will not need to decorate it with additional attributes, etc., to serialize it. Similarly, if ORM works with vanilla types, it will not need to implement a specific interface or something like that.
EDIT: Well, seeing the context, the above seems to be correct.
Another way to think about it is that you can write a type that can then be used by the framework without knowing the type of structure. Thus, some DI frameworks require you to decorate types yourself in order to allow injection, while others do not. Recent structures can work with vanilla types.
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