Follow https://github.com/Microsoft/visualstudio-docs/blob/master/docs/extensibility/faq-2017.md#can-i-build-a-vsix-v3-with-visual-studio-2015

Modify .csproj for proper building of .vsix

ILSpy.AddIn/ILSpy.AddIn.csproj

@@ -1,14 +1,11 @@
 <?xml version="1.0" encoding="utf-8"?>
 <Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003" ToolsVersion="12.0">
-  <PropertyGroup>
+  <Import Project="..\packages\Microsoft.VisualStudio.Sdk.BuildTasks.14.0.14.0.23-pre\build\Microsoft.VisualStudio.Sdk.BuildTasks.14.0.props" Condition="Exists('..\packages\Microsoft.VisualStudio.Sdk.BuildTasks.14.0.14.0.23-pre\build\Microsoft.VisualStudio.Sdk.BuildTasks.14.0.props')" />
-    <!-- Use the SDK for the current version of Visual Studio -->
+    <PropertyGroup>
-    <VsSdkTargets Condition="'$(VisualStudioVersion)'!=''">$(MSBuildExtensionsPath)\Microsoft\VisualStudio\v$(VisualStudioVersion)\VSSDK\Microsoft.VsSDK.targets</VsSdkTargets>
+    <MinimumVisualStudioVersion>14.0</MinimumVisualStudioVersion>
-    <VsSdkTargets Condition="'$(VisualStudioVersion)'==''">$(MSBuildExtensionsPath)\Microsoft\VisualStudio\v10.0\VSSDK\Microsoft.VsSDK.targets</VsSdkTargets>
+    <VSToolsPath Condition="'$(VSToolsPath)' == ''">$(MSBuildExtensionsPath32)\Microsoft\VisualStudio\v$(VisualStudioVersion)</VSToolsPath>
-    <TargetFrameworkProfile />
+    <NuGetPackageImportStamp>
-  </PropertyGroup>
+    </NuGetPackageImportStamp>
-  <PropertyGroup Condition="'$(VisualStudioVersion)' != ''">
-    <!-- This is added to prevent forced migrations in Visual Studio 2012 and newer -->
-    <MinimumVisualStudioVersion>$(VisualStudioVersion)</MinimumVisualStudioVersion>
   </PropertyGroup>
   <Import Project="$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props" Condition="Exists('$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props')" />
   <PropertyGroup>
@@ -22,6 +19,7 @@
     <RootNamespace>ICSharpCode.ILSpy.AddIn</RootNamespace>
     <AssemblyName>ILSpy.AddIn</AssemblyName>
     <SignAssembly>True</SignAssembly>
+	<VsixType>v3</VsixType>
     <AssemblyOriginatorKeyFile>Key.snk</AssemblyOriginatorKeyFile>
     <TargetFrameworkVersion>v4.0</TargetFrameworkVersion>
   </PropertyGroup>
@@ -235,7 +233,15 @@
     <UseCodebase>true</UseCodebase>
   </PropertyGroup>
   <Import Project="$(MSBuildBinPath)\Microsoft.CSharp.targets" />
-  <Import Condition="Exists($(VsSdkTargets))" Project="$(VsSdkTargets)" />
+  <Import Project="$(VSToolsPath)\VSSDK\Microsoft.VsSDK.targets" Condition="'$(VSToolsPath)' != ''" />
+  <Target Name="EnsureNuGetPackageBuildImports" BeforeTargets="PrepareForBuild">
+    <PropertyGroup>
+      <ErrorText>This project references NuGet package(s) that are missing on this computer. Use NuGet Package Restore to download them.  For more information, see http://go.microsoft.com/fwlink/?LinkID=322105. The missing file is {0}.</ErrorText>
+    </PropertyGroup>
+    <Error Condition="!Exists('..\packages\Microsoft.VisualStudio.Sdk.BuildTasks.14.0.14.0.23-pre\build\Microsoft.VisualStudio.Sdk.BuildTasks.14.0.props')" Text="$([System.String]::Format('$(ErrorText)', '..\packages\Microsoft.VisualStudio.Sdk.BuildTasks.14.0.14.0.23-pre\build\Microsoft.VisualStudio.Sdk.BuildTasks.14.0.props'))" />
+    <Error Condition="!Exists('..\packages\Microsoft.VisualStudio.Sdk.BuildTasks.14.0.14.0.23-pre\build\Microsoft.VisualStudio.Sdk.BuildTasks.14.0.targets')" Text="$([System.String]::Format('$(ErrorText)', '..\packages\Microsoft.VisualStudio.Sdk.BuildTasks.14.0.14.0.23-pre\build\Microsoft.VisualStudio.Sdk.BuildTasks.14.0.targets'))" />
+  </Target>
+  <Import Project="..\packages\Microsoft.VisualStudio.Sdk.BuildTasks.14.0.14.0.23-pre\build\Microsoft.VisualStudio.Sdk.BuildTasks.14.0.targets" Condition="Exists('..\packages\Microsoft.VisualStudio.Sdk.BuildTasks.14.0.14.0.23-pre\build\Microsoft.VisualStudio.Sdk.BuildTasks.14.0.targets')" />
   <!-- To modify your build process, add your task inside one of the targets below and uncomment it. 
        Other similar extension points exist, see Microsoft.Common.targets.
   <Target Name="BeforeBuild">

ILSpy.AddIn/packages.config

@@ -5,6 +5,7 @@
   <package id="EnvDTE80" version="8.0.1" targetFramework="net40" />
   <package id="EnvDTE90" version="9.0.1" targetFramework="net40" />
   <package id="EnvDTE90a" version="9.0.1" targetFramework="net40" />
+  <package id="Microsoft.VisualStudio.Sdk.BuildTasks.14.0" version="14.0.23-pre" targetFramework="net40" developmentDependency="true" />
   <package id="stdole" version="7.0.3301" targetFramework="net40" />
   <package id="VSSDK.DTE" version="7.0.4" targetFramework="net40" />
   <package id="VSSDK.IDE" version="7.0.4" targetFramework="net40" />

ILSpy.AddIn/source.extension.vsixmanifest

@@ -21,4 +21,7 @@
   <Assets>
     <Asset Type="Microsoft.VisualStudio.VsPackage" d:Source="Project" d:ProjectName="%CurrentProject%" Path="|%CurrentProject%;PkgdefProjectOutputGroup|" />
   </Assets>
+  <Prerequisites>
+    <Prerequisite Id="Microsoft.VisualStudio.Component.CoreEditor" Version="[15.0,16.0)" DisplayName="Visual Studio core editor" />
+  </Prerequisites>
 </PackageManifest>