If I'm understanding this correctly, it should be at least as safe as NIP-46 if you hold your own shard and require all shards to open the vault. (I guess that also assumes that the underlying cryptography is as good as when you just hold the full key.)
If you have your own vault, is the experience of that similar to having a remote signer? Can one have a vault on one's phone, maybe something that can be used like Amber?