A Visual Studio Installer (vsi) is a zip file renamed to vsi. The internal structure of the zip should be:

\
 VSTemplate.vscontent
 Template1.zip
 Template2.zip

It may contain more than one template. The VSTemplate.vscontent is the manifest of the vsi and will describe what templates will be installed under what category

<VSContent xmlns="http://schemas.microsoft.com/developer/vscontent/2005">
  <Content>
    <FileName>WPFCABShellTemplate.zip</FileName>
    <DisplayName>WPF CAB Shell Application Template (SmartClient Contrib)</DisplayName>
    <Description>WPF CAB Shell Application Template (SmartClient Contrib)</Description>
    <FileContentType>VSTemplate</FileContentType>
    <ContentVersion>1.0</ContentVersion>
    <Attributes>
      <Attribute name="TemplateType" value="Project"></Attribute>
      <Attribute name="ProjectType" value="Visual C#"></Attribute>
      <Attribute name="ProjectSubType" value="Windows"></Attribute>
    </Attributes>
  </Content>
</VSContent>

For each template a <Content> element must be added.
The FileName element points to the zipped template.

To build the vsi select the templates zip files and the .vscontent and "Send To -> Compressed (zipped) folder". Rename the generated zip file to .vsi.