ASP.NET Web Form execute after Composite C1 has constructed the page so the you are in control at this point (the time for normalization of nested xhtml documents that the Composite C1 rendering engine does for xslt, mvc, etc. has passed).
If ViewModel.Content is maintained by the visual html editor inside the C1 Console this will contain a complete xhtml document - in that case you should strip away anything but the content of the body element.
If you never expect users to actually put anything into the <head /> section of that particular content I expect the best performing way would be to locate "<body>" and "</body>" inside this string, and then grab the
substring in between.