If we take a look at our project configuration file--the projectName.csproj in the project's root folder--we can see that the Webpack vendor configuration file is built in debug mode only if the /wwwroot/dist/ folder doesn't exist:

[...]

<Target Name="DebugRunWebpack" BeforeTargets="Build" Condition=" '$(Configuration)' == 'Debug' And !Exists('wwwrootdist') ">
  <!-- Ensure Node.js is installed -->
  <Exec Command="node --version" ContinueOnError="true">
    <Output TaskParameter="ExitCode" PropertyName="ErrorCode" />
  </Exec>
  <Error Condition="'$(ErrorCode)' != '0'" Text="Node.js is required to build and run this project. To continue, please install Node.js from https://nodejs.org/, and then restart your command prompt or IDE." />

  <!-- In development, the dist files won't exist on the first run or when cloning to
       a different machine, so rebuild them if not already present. -->
  <Message Importance="high" Text="Performing first-run Webpack build..." />
  <Exec Command="node node_modules/webpack/bin/webpack.js --config webpack.config.vendor.js" />
  <Exec Command="node node_modules/webpack/bin/webpack.js" />
</Target>

[...]

Therefore, we can just delete the /dist/ folder to trigger the build.