Add bold formatting to localized string that has placeholders

Question

I have a localization string with a placeholder:

Verb {0}

I use this string in my view-model to return a string to my view that, in turn, is displayed in a TextBlock. Easy enough. But a new requirement has arisen saying that the "Verb" portion (everything other than the placeholder's inserted value) be displayed in bold.

Using a string with placeholders seems like the typical and easiest way to indicate word order. So the first question, then, is: where should I parse the localization string in order to add the bold formatting? The parse operation will need knowledge of the original placeholder's location. So far, the view-model has been responsible for utilizing the localization strings by using string.Format to insert values and return its result to the view. If I leave this responsibility in the view-model, as is probably necessary, then the view-model also needs to return rich text.

Is binding to rich text even supported by RichTextBlock? Even if it is supported, I've never before had a view-model return formatted text before. It initially feels sacrilegious to a follower of MVVM-ism, but perhaps upon further consideration I may find it acceptable.

What's the best way to add bold formatting to a localized string that has placeholders? Is returning rich text from the view-model the best way?