"Viola!" may be a bit of a cheesy word play joke.
A viola is a stringed instrument, but because the spelling is so similar to the French word voilà, it is sometimes used as a substitute, either intentionally as self-deprecating humor (implying that the speaker chooses to use the wrong, but similar-sounding word, as a sort of pun), or unintentionally (which may imply that the speaker learned the word through reading, rather than hearing it in use, as the letter transposition is much easier to miss while reading printed text than it would be in hearing the word properly pronounced).
I know a couple of people who use "viola" in real life as an intentionally (and arguably) humorous substitute for "voilà", and I know that they are fully aware of the differences.
However, there are plenty of people who unintentionally misuse viola, or who pronounce it incorrectly (e.g. "wallah").
Given the level of vocabulary Vernor Vinge demonstrates throughout his writings, it seems unlikely that he is unaware of the distinction between viola and voilà. His use of the incorrect term is almost certainly deliberate.
Whether Mr. Rabbit is aware of the distinction, however, is a bit less clear.
Given that the actual nature of Mr. Rabbit is uncertain, this particular letter transposition is rather interesting.
An unintentional letter transposition is pretty much unimaginable, should Mr. Rabbit prove to be an emergent AI.
However, given how fond Mr. Rabbit is of continually bragging about his skills, abilities, and general awesomeness, self-deprecating humor seems a bit out of character. It's been a couple of months since I read it, but I seem to recall Mr. Rabbit boasting each and every chance he got (which was essentially every time he spoke). Making a pun that implies he gets confused on obscure words does not fit in with the rest of his personality.
Which leaves two possibilities:
This is a hint that he is not, in fact, an emergent AI, but rather an extraordinarily intelligent human who perhaps learned a significant portion of their language skills via reading, instead of voice interaction.
This is a red herring that the emergent AI uses in order to suggest that it is not an AI.
Given that one of the three uses of the term was during what appears to be an inner monologue, I'm leaning towards it being an indication that perhaps Mr. Rabbit really isn't an emergent AI. However, this is far from conclusive.