The output templates (.vbdt files) are just plain text files with a simple macro syntax. You can edit them in any text editor, including Visual Studio.

The .vbdt files use very simple syntax but the entire template code may look quite messy. It may be difficult to navigate in it, if there's no syntax highlighting.

Starting with Visual Studio 2015, you can create custom syntax highlighters for your own file types. This is thanks to the Visual Studio Extension for TextMate Grammars which is a part of the VS installation by default. VSdocman comes with the syntax highlighter for .vbdt files. If you need to customize the templates, you can install the highlighter and then edit the .vbdt files in Visual Studio comfortably.

To install the .vbdt grammar:

1.Close Visual Studio.

2.Go to C:\Program Files (x86)\VSdocman\Templates\Vbdt editing\ folder. There's vbdt subfolder inside it.

3.Copy the vbdt subfolder to C:\Users\<USERNAME>\.vs\Extensions\ folder. This is the folder with the TextMate extension grammars.

Note: If you have the Syntax Highlighting Pack extension installed in your Visual Studio, it will rewrite the C:\Users\<USERNAME>\.vs\Extensions\ folder. To make the grammar work, you need to do the following:
Find the Syntax Highlighting Pack extension's directory in the local AppData folder. The extensions root folder should be like C:\Users\<USERNAME>\AppData\Local\Microsoft\VisualStudio\14.0\Extensions\, where 14.0 is for VS 2015 and 15.0_XY is for VS 2017.
There are many subfolders with cryptic names, find the correct one by searching for TextmateBundleInstaller.pkgdef file. For example, you'll find it in C:\Users\<USERNAME>\AppData\Local\Microsoft\VisualStudio\14.0\Extensions\htmrdfpe.z35. Copy the vbdt subfolder with the grammar to Bundles subfolder in the folder you found, e.g. to: C:\Users\<USERNAME>\AppData\Local\Microsoft\VisualStudio\14.0\Extensions\htmrdfpe.z35\Bundles\.

4.Delete languages.cache and (if exists) Syntax Highlighting Pack.log files in C:\Users\<USERNAME>\.vs\Extensions\.

Now you can start Visual Studio, open a .vbdt file and you should see its syntax highlighted.

Note: In VS 2015, the Visual Studio Extension for TextMate Grammars doesn't support VS color schemes. In VS 2017, this is supported and the colors from the VS settings are used.