Can I install a SSMS extension with the VSIX installer

Is it possible to install an SSMS extension by using a .vsix file?  In my VS project I have a source.extension.vsixmanifest file:

<?xml version="1.0" encoding="utf-8"?>
<Vsix xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" Version="1.0.0" xmlns="http://schemas.microsoft.com/developer/vsx-schema/2010">
  <Identifier Id="56f92f34-58ad-4185-8b21-a832de1b3559">
    <Name>MySSMSAddin VSIX manifest</Name>
    <Author>My Software Company</Author>
    <Version>15.0</Version>
    <Description xml:space="preserve">My SSMS Addin for SQL Server Management Studio 2018</Description>
    <Locale>1033</Locale>
    <InstalledByMsi>false</InstalledByMsi>
    <SupportedProducts>
      <IsolatedShell Version="15.0">ssms</IsolatedShell>
    </SupportedProducts>
    <SupportedFrameworkRuntimeEdition MinVersion="4.0" MaxVersion="4.0" />
    <SystemComponent>true</SystemComponent>
  </Identifier>
  <References />
  <Content>
    <VsPackage>MySSMSAddin2018.pkgdef</VsPackage>
  </Content>
</Vsix>

This gets built into a MySSMSAddin2018.vsix file.  But when I execute this .vsix file the VSIX Installer displays a message box, stating:

   The install of CvSSMSAddin VSIX manifest" was not successful for all of the selected products.  For more information, click on the install log link at the bottom of the dialog.

I have 3 different versions of SSMS installed on my system.  I want to install my extension on SQL Server Management Studio version 18.1 (internal version 15.0).  I assume that the message box was telling me that I cannot install on the other two SSMS versions because it did successfully install the files in the .vsix into C:\Users\RonaldWright\AppData\Local\Microsoft\SQL Server Management Studio\18.0\Extensions\qgmpi40j.ccu\.  But even with this apparent install, the resulting MySSMSAddin2018.pkgfile still has some path macros ($WinDir$ and $PackageFolder$) which were not expanded.  Also, when I launch SSMS.exe (the 18.1) version, SSMS does not attempt to load the MySSMSAddin2018.pkgdef file.

Is there a resolution to this problem?

Thanks for the outstanding help.