The fact that it was not possible to route traffic on a Crostini container over a VPN has been one of the most annoying bugs for developers who wish to use their Chromebooks to work in a corporate environment. Google has been working on a fix since I reported it over a year ago. The community of Chromebook owners were saddened to report that the fix was pushed back to M76 about 2 weeks ago. It looks like Google is going to squeeze it into M75, specifically in build 12104.0.0 which may hit the developer channel any day now.
Edit : Its LIVE!!!
Aside from developers and remote workers, other users of Crostini will benefit from this change, especially if they commonly use a public internet connection, such as in a coffee shop to work. The use of a VPN significantly improves security, may provide better privacy and can get around some network restrictions.