Это можно упростить так:
<!DOCTYPE HTML>
<html>
<head> </head>
<body>
<iframe id="player" height="249" scrolling="no" width="320" src="https://www.youtube.com/embed/vUmLfvHkGus"></iframe>
<br>
<button onclick="document.getElementById('player').src='https://www.youtube.com/embed/vUmLfvHkGus'">Player 1</button>
<button onclick="document.getElementById('player').src='https://vk.com/video_ext.php?oid=11767786&id=169403336&hash=a1a5d2c281b53598&hd=2'">Player 2</button>
</body>
</html>
или так:
<!DOCTYPE HTML>
<html>
<head> </head>
<body>
<iframe id="player" height="249" scrolling="no" width="320" src="https://www.youtube.com/embed/vUmLfvHkGus"></iframe>
<br>
<button onclick="player.src='https://www.youtube.com/embed/vUmLfvHkGus'">Player 1</button>
<button onclick="player.src='https://vk.com/video_ext.php?oid=11767786&id=169403336&hash=a1a5d2c281b53598&hd=2'">Player 2</button>
</body>
</html>