"Complete" is certainly in the eyes of the beholder, but Viper's author seems to think the language is "complete enough" to have started writing Ethereum's new Proof-of-Work/Proof-Of-Stake hybrid system (aka Casper contracts[1]) in Viper[2][3].
[1] https://www.ethnews.com/proof-of-stake-vitalik-buterin-share...
[2] https://ethereumclassic.github.io/blog/2017-03-13-viper/
[3] https://github.com/ethereum/casper/blob/master/README.md