@humblecoder You should be able to use “pure” cordova, indeed. But then, you are trying to run Monaca commands here, right? There is a Monaca guide here for this specific template. This link (and more info) is shown in the terminal right after vue init OnsenUI/vue-cordova-webpack command finishes but I guess you didn’t see it.
Vue CLI templates are a bit different from the rest of Monaca templates, so instead of monaca preview you’ll need to run npm run dev (it’s how Vue CLI templates work by default). I think all the necessary stuff is explained in that guide anyway 
For creating projects without templates, well, you basically need to know how to create a Vue project. Onsen UI is just a plugin for it and you can see how to include it in the docs. After that, if you want your Vue app to be Cordova-like, have a look at their project structure guide. The most important things are config.xml and www folder (with all the public files). The rest is using Cordova CLI to add platforms and plugins, or Monaca CLI.
Therefore, what the templates provide is basically a Vue app with Onsen UI imported and a config.xml file. The build process has been also a bit changed to produce a www folder instead of the default dist folder.
Edit: I just tried with Cordova CLI and the problem is the same, it simply needs www so you need to run npm run build.
