I can get you started:
Visual Studio expects XML:
https://learn.microsoft.com/en-us/visualstudio/ide/walkthrough-creating-a-code-snippet
Visual Assist stores its snippets in a legacy, non-XML format. Fortunately, you can "export" XML using the VA Snippet Editor. Open the editor; multi-select in the list; copy; paste into an external editor.
If the snippets you want to export share words in their descriptions or follow a pattern, you can simplify the multi-select by limiting what appears in the list. Change the Type dropdown to "Search", then search a field by string or regex.
I don't have a convenient way to convert from one XML format to the other.