Unfortunately you cannot do it within a partial view.
The reason for this is by the time your partial view gets parsed by the view parser, the main layout (which contains the <title></title>
tags) has already been written to your applications response stream ready to be flushed to the browser.
If you REALLY need to then I suppose you could parse the current response stream and use regular expression to match and replace the page title, but I wouldn't suggest this as it's pretty inefficient.
As others have said, your better option (albeit not ideal - one reason being the importance a title tag is to search engines) is set the title using Javascript.
This can be achieved like so:
<script type="text/javascript">
document.title = '@ViewBag.Title';
</script>