Loading (well, reloading) workflows that include Visual Basic expressions in a WCF hosted service is not as straightforward as one might expect. If you have created your XAML workflow based on an ActivityBuilder or a DynamicActivity then the Visual Basic settings aren’t configured automatically.
One of the clear advantages of using Visual Studio for editing workflows over a custom implementation is the readily available auto-completion features of the IDE. A similar style of auto-completion can be added to your re-hosted workflow designer by implementing your own ‘editor service’ and publishing it to the workflow designer control.