What's the least the French could be made to settle for under the circumstances?
What are the odds of them seeking revanche again by 1900? [although France started the war, colonial powers in those days felt compelled to get "revenge" not only for attacks on themselves but for occasions when their expansion was frustrated {see history of Italy and Ethiopia}]
I'm on my phone so can't do as full an answer as I'd like but I doubt you'd get revanchist sentiment as a full on defeat is absolutely impossible at that time.
However, you can get France throwing the towel, feeling it's not worth it.
The thing is, the war against China is really a war against Vietnam with Chinese intervention, so you'd still get a puppet emperor in Hué and a subjugation of the country.
France would not sever the official tributary link between China and Vietnam, but it doesn't matter much.
If French hold over Tonkin is lighter, and the tributary link still there, they'd probably annex Annam outright instead of maintaining the spectacle of the protectorate. No need for a tributary link if there's no Vietnamese state after all.
Risk is with guerilla and a proper court in exile but France would not accept an uppity colony