It does feel too soon for Vrabel, but there are precedents for guys turning into decent coaches without dominating as coordinators.
I'm not a fan of Tomlin or Harbaugh, but both have winning records and have been around a while despite not having great track records as coordinators.
Tomlin was Tampa's DB coach for a few years with Dungy, then served 1 year as the DC for Minnesota when they finished 14th in points allowed (8th in yards allowed to be fair), then beat out two internal Steelers candidates (Whisenhunt, Grimm) to surprisingly get the job.
Harbaby was a special teams coach for several years, then got a chance to coach the DBs for one year, where the Eagles finished 18th in passing yards allowed. But he still got the job with the Ravens the next year, largely due to Jason Garrett turning down the job and Belichick highly recommending Harbaugh to owner Steve Bisciotti.
I think it will be tough for Vrabel, but not impossible. I worry that he's too far behind with the assistant coaches, as many of the top names have already found new homes. He should hire or keep veteran coordinators, perhaps some with previous HC experience. I feel like Sean McVay really benefited from having Wade Phillips as his DC, not just on the defensive side but also as a mentor to help him adjust to the head coaching role. Vrabel would be wise to do something similar.
