This issue is not relevant to the Office 365 Add-in.
If you have tried to install the PowerPoint Add-in and received an error message stating ‘The download of Microsoft VSTO Runtime failed’, make sure to first download the Microsoft VSTO Runtime file before uploading the Vevox PowerPoint Add-in again.
Microsoft VSTO Runtime is a pre-requisite Microsoft software and is required for all Microsoft Add-ins to work.
To fix this error message, follow these steps:
Download Microsoft VSTO Runtime by clicking on the link below and save it in your applications folder: https://www.microsoft.com/en-GB/download/details.aspx?id=48217
Once you have successfully downloaded the Microsoft VSTO Runtime file, you will now be able to download the Vevox PowerPoint Add-in. Open your Vevox dashboard again. The PowerPoint Add-in option appears in the main navigation menu on the left. Select PowerPoint Add-in and click on it.
Now follow from step 4 onwards in this article: https://help.vevox.com/hc/en-us/articles/360011104118