Hi Rick,
VSTO add-in is version-sensitive, which means that add-ins for Office 2007 don't work for Office 2010. Installing Visio 2010 will modify the registry, which causes that the target machine identified as Office 2010 installed.
http://blogs.msdn.com/b/andreww/archive/2007/06/08/why-is-vs-development-not-supported-with-multiple-versions-of-office.aspx
You can try to fix the issue by migrate your add-in to Outlook 2010 and deploy it in such machines.
I hope this helps.
Best Regards, Calvin Gao [MSFT]
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