Also worth noting that while vibe.d does not provide some convenience functions Mongo shell does, MongoDB protocol is designed in such way that almost any possible query can be reduced to find(). One can always sniff the request from shell by wireshark and reconstruct it manually in D code, see https://github.com/rejectedsoftware/vibe.d/issues/295 for example.
(You may already know this but it is a common newbie issue)