It depends. If the point of the VC money is to go after enterprise customers and to expand into other enterprisey software security products, then $100M seems reasonable to me, especially for the time when the investment happened. The VC market seems to have cooled quite a bit from when Bitwarden took that investment, so times change and maybe they were just striking while the iron was hot?

The $10/year individual plan wouldn't warrant $100M investment. But going after big companies who are going to commit to $X/year/employee or similar kinds of pricing packages might, especially if Bitwarden integrates with existing corporate directory systems and such for delegating and managing accounts.