There’s no floor on team performance, but Vrabel has demonstrated that he’s a competent head coach; Johnson hasn’t. The “floor” for Johnson is Jerod Mayo 2.0; the floor for Vrabel is higher than that. Shouldn’t be a major consideration, but could be a tiebreaker if you’re undecided between the two.
Your larger point is correct (Vrabel more of a known quantity with a proven track record), but Johnson this year and Mayo last year are not really similarly situated. Johnson is a highly sought after coaching candidate who is currently one of the best offensive coordinators in the NFL.
Johnson has 13 years of experience as an NFL assistant coach and 3 years as a college assistant before that. In the NFL he has been an offensive assistant, assistant QB coach, TE coach, assistant WR coach, WR coach, offensive QC coach, passing game coordinator (1 year), OC (3 years).
Jerod Mayo, by contrast, had 5 years as an inside linebackers coach in the NFL before the Pats gave him the head coach job last year. Given the wierdness of the way the Pats hand out coaching titles, it is reasonable to think he had more reasonaibilities as an assistant coach than the title would suggest. In 2023, he was a sort of co-defensive coordinator of a good defense. But we know he was not the playcaller and we know that BB is a defensive guru who probably hads a lot to do with that success.
All of which is to say... Mayo was virtually assured to fail and Johnson is not.
A better downside comp for Johnson is probably Josh McDaniels. He had 5 years with the Pats in various assistant coaching roles before becoming OC, which he did for 3 years, before getting his first head coaching job, where he faceplanted.
Anyway, personally I want Vrabel but I am not going to be upset if the hire Johnson.