Since I can add an external code such as codepen or jsfiddle in the markdown look in the documentation but I did not find it very much thanks.
Codepen support sharing as an embed iframe:

I just tested for it. it works for now.
If you just want to demo Vue code with markdown, you can using vue at markdown instead.
Sorry for the inconvenience you could show me the code to add the iframe in the markdown
Just... use an iframe? Look for it in Codepen's docs?
Excuse me, I just forget to give your the test iframe code:
<iframe height='265' scrolling='no' title='OZXQWp' src='//codepen.io/ulivz/embed/OZXQWp/?height=265&theme-id=0&default-tab=html,result&embed-version=2' frameborder='no' allowtransparency='true' allowfullscreen='true' style='width: 100%;'>See the Pen <a href='https://codepen.io/ulivz/pen/OZXQWp/'>OZXQWp</a> by ULIVZ (<a href='https://codepen.io/ulivz'>@ulivz</a>) on <a href='https://codepen.io'>CodePen</a>.
</iframe>
You can find the embed button at the right bottom side in codepan.
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Excuse me, I just forget to give your the test iframe code:
You can find the embed button at the right bottom side in codepan.