I don't know how active the Maker userbase is, but Augur has 700 monthly active users.
It seems like it would be possible to manipulate the validator set for a particular shard - also, isn't part of the point of ethereum to be able to access information from other contracts? How do you guarantee that that contract is on the same shard?
Augur just launched a couple weeks ago.
Preventing validator set manipulation is an important part of sharding research. Part of the solution is getting good random numbers that can't be manipulated, and there are several approaches for that.
Cross-shard communication gets complicated. There are some ideas for it, but that's several years down the road. It'll still be a big step up to have lots of shards, each with the capacity of the full blockchain today, sharing the security of all of them but not getting swamped with traffic if something gets really popular on another shard.
But it wasn't Vitalik who created that; what he did was extend it into an application platform. That platform is working, with live applications including decentralized exchanges, Maker (stable currency and collateralized loans) and Augur (prediction market with decentralized bet resolution).
Sharding is one of the main things he and the research team are working on now (along with proof of stake). The current design uses 4000 shards, so unless you consider 1/4000 to be a large subset, it would answer your objection if they manage to get it working.