The purse deposit can fail in case of overflow. In such situation we need to revert the split.
Yeah, we should at least change the message. I'm not sure though what should happen when one transfers to an already-full vault:
should the funds be lost (easiest one)
should they be refunded (can cause a cascade and is subject to concurrency bugs)
should the vault keep a pile of purses and prevent overflow proactively? (require substantial development effort)
Given that it's unclear how to proceed, and that this is admittedly a rare scenario - are you OK with me scheduling a JIRA for fixing that later?