function wait with return until $.getJSON is finished

you can simple use $.getJSON's callback like following:

function result(res) {
  console.log(res);
}

function getThumbnail(vUrl) {
   var thumbnail   = '';
   var title       = '';
   var caption     = '';
   var content     = '';

   $.getJSON("http://api.embed.ly/1/oembed?key=:key&url="+vurl, function(data) {
     var thumbnail = data.thumbnail_url;
     console.log(thumbnail);

     var result = {
        thumbnail:thumbnail,
        vurl:vurl
      };

     // passing the result to a function
     getResult(result);

   });
}

NOTE:

You see that I'm calling a function to pass the result, where you are trying to return, but you can't return result to caller function. Because, $.getJSON is asynchronous.

Using Async/Await

async function getThumbnail(vUrl) {
  const data = await $.getJSON("http://api.embed.ly/1/oembed?key=:key&url="+vUrl);
  return {
      thumbnail:data.thumbnail_url,
      vurl:vUrl
    }
}

async function someFunction() {
    let thumbNail = await getThumbnail('the_vurl_');
}

Updated answer

getJSON returns a promise (a read-only deferred), so you can listen to it. But since you need some post-processing, you'd want to chain a then which allows you to alter the resolved value.

// Now using `then`
function getThumbnail(vUrl){
  return $.getJSON("http://api.embed.ly/1/oembed?key=:key&url="+vurl).then(function(data){
    return {
      thumbnail:data.thumbnail_url,
      vurl:vurl
    }
  });
}

//and in your call will listen for the custom deferred's done
getThumbnail('the_vurl_').then(function(returndata){
  //received data!
});

---

Original answer

You can use a deferred object, and listen for the done().

function getThumbnail(vUrl) {
    //create our deferred object
    var def = $.Deferred();

    //get our JSON and listen for done
    $.getJSON("http://api.embed.ly/1/oembed?key=:key&url="+vurl)
        .done(function(data){

            //resolve the deferred, passing it our custom data
            def.resolve({
                thumbnail:data.thumbnail_url,
                vurl:vurl
            });
        });

    //return the deferred for listening
    return def;
}

//and in your call will listen for the custom deferred's done
getThumbnail('the_vurl_')
    .done(function(returndata){
        //received data!
    });

You could return $.getJSON's deferred to get the raw data. But because of "post-processing" into an object, the custom deferred is needed. You could also pass a callback to getThumbnail():

function getThumbnail(vUrl,callback) {
    $.getJSON("http://api.embed.ly/1/oembed?key=:key&url="+vurl,function(returndata){
        callback(returndata);
    });
}

getThumbnail('the_vurl_',function(returndata){
    //received data!
})