Introduction

If you want to know how to compile and debug VSTO SE Outlook 2003 Addin on a computer with the configuration: Vista+Office 2007, read this article.

Steps

Install Office 2003 PIA and Office 2007 PIA on the same computer

Install Office 2003 in trial mode
Install Office 2003 PIA (we need 1. to install 2003 PIA)
Uninstall Office 2003
Install Office 2007
Create 2003 Addin in Visual Studio project
Change 2003 COM reference to 2003 PIA in .csproj for "none error" compilation

Open project file in text editor and find this fragment:

<!--
This section specifies COM References for the project 
(managed assemblies that wrap unmanaged typelibs (tlb)). 
This is the equivalent of choosing "Add Reference->Com Reference" 
in the IDE.
-->
<ItemGroup>
    <COMReference Include="Microsoft.Office.Core">
        <Guid>{2DF8D04C-5BFA-101B-BDE5-00AA0044DE52}</Guid>
        <VersionMajor>2</VersionMajor>
        <VersionMinor>3</VersionMinor>
        <WrapperTool>primary</WrapperTool>
    </COMReference>
    <COMReference Include="Outlook">
        <Guid>{00062FFF-0000-0000-C000-000000000046}</Guid>
        <VersionMajor>9</VersionMajor>
        <VersionMinor>2</VersionMinor>
        <WrapperTool>primary</WrapperTool>
    </COMReference>
    <COMReference Include="stdole">
        <Guid>{00020430-0000-0000-C000-000000000046}</Guid>
        <VersionMajor>2</VersionMajor>
        <VersionMinor>0</VersionMinor>
        <Lcid>0</Lcid>
        <WrapperTool>primary</WrapperTool>
        <Isolated>False</Isolated>
    </COMReference>
</ItemGroup>

Replace found fragment with:

<!--
This section specifies COM References for the project 
(managed assemblies that wrap unmanaged typelibs (tlb)). 
This is the equivalent of choosing "Add Reference->Com Reference" 
in the IDE.
-->
<ItemGroup>
    <Reference Include="Microsoft.Office.Interop.Outlook, 
        Version=11.0.0.0, Culture=neutral, 
        PublicKeyToken=71e9bce111e9429c" />
    <Reference Include="Office, Version=11.0.0.0, 
        Culture=neutral, PublicKeyToken=71e9bce111e9429c" />
    <Reference Include="stdole, Version=7.0.3300.0, 
        Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a">
    <SpecificVersion>False</SpecificVersion>
    </Reference>
</ItemGroup>

Save and now we have a 2003 Addin, which may be compiled under Vista+2007 Office and may be run under 2003 Office.
Note: Be careful, "do not" set <SpecificVersion>False</SpecificVersion> for Office and Outlook DLL, because if you do it, your Addin will not run under 2003 Office.
Turn on Debug Capability

Now, all is fine, but you can't debug 2003 Addin under 2007 Office. So for debug capability, in the project file, you need to change code which is in the tag <ProjectExtensions> with fragment code from Outlook 2007 Addin project.

<!-- 
This section defines VSTO properties that describe 
the host-changable project properties. 
--> 
<ProjectExtensions> 
  <VisualStudio> 
    <FlavorProperties GUID="{BAA0C2D2-18E2-41B9-852F-F413020CAA33}"> 
      <ProjectProperties HostName="Outlook" 
    HostPackage="{D53BAEDE-5B63-42BE-8267-3DED11EDC582}" 
    ApplicationType="Outlook" Language="cs" TemplatesPath="" 
    DebugInfoExeName=
   "#Software\Microsoft\Office\12.0\Outlook\InstallRoot\Path#Outlook.exe" 
    AddItemTemplatesGuid="{147FB6A7-F239-4523-AE65-B6A4E49B361F}" /> 
        <Host Name="Outlook" 
        GeneratedCodeNamespace="OutlookAddIn3" IconIndex="0"> 
        <HostItem Name="ThisAddIn" Code="ThisAddIn.cs" 
        CanonicalName="ThisAddIn" CanActivate="false" 
        IconIndex="1" Blueprint="ThisAddIn.Designer.xml" 
        GeneratedCode="ThisAddIn.Designer.cs" /> 
        </Host> 
    <ProjectClient> 
      <VSTO_CompatibleProducts ErrorProduct=
      "This project requires Microsoft Office Outlook 2007, 
      but this application is not installed." 
      ErrorPIA="This project references the primary interop assembly 
      for Microsoft Office Outlook 2007, but this primary interop 
      assembly is not installed."> 
      <Product Code="{XX12XXXX-XXXX-XXXX-X000-X000000FF1CE}" 
      Feature="OUTLOOKFiles" PIAFeature="Outlook_PIA" /> 
      </VSTO_CompatibleProducts> 
      </ProjectClient> 
    </FlavorProperties> 
  </VisualStudio> 
</ProjectExtensions>

Developing Outlook 2003 Addin on VSTO SE under Vista + 2007 Office