So, VirtualURIFilter cannot be used on multilanguage sites.
The locale is set on the aggregationstate by the I18nContentSupportFilter, it is determined by the active I18nContentSupport. When DefaultI18nContentSupport is used, it determines the locale from the aggregationState currentURI. (A benefit of the currentURI is that it already has the site removed from the uri)

If a VirtualURIFilter matches the request, it can perform a forward.

This will cause the filter chain to be run again but this time when I18nContentSupport is hit, the URI no longer contains the locale, so the fallback locale will be set on the aggregation state.

So the problem is that I18nContentSupport gets run twice on the request, but it is only designed to be run once.

See related ticket for example.