The way I understand it, Joe Vitt would otherwise be the interim HC if anything ever happened to Payton based on the team's contingency plans. Vitt's suspension is 6 games but I don't believe it comes into effect until after the preseason.
I'm not sure how they'll handle it, but Vitt would be available to be HC for everything except weeks 1 - 6. But perhaps that's a substantial enough period of time that its better to go ahead and make Carmichael (or someone else) HC for the whole season. And Vitt will be assistant HC when he returns.