MVC3 validate only one entity in ViewModel?

What you have shown is not a view model. You call it a view model but it isn't because it is referencing your EF domain entities.

Instead of putting the entities directly in the view model, put the properties for the entities in the view model and map between the view model and the actual entity objects on the server. That way you can control what properties are required for your view. Create some custom validation rules to validate that the required company properties are there when some company information is required.

To do this on the server, you can have your view model implement IValidatableObject and implement the logic in the Validate method. On the client you can add rules to the jQuery validate plugin for the "required if" properties.

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