One other reason to leave the 401k as is (even if you move it to Vanguard) is for protection from lawsuits/judgements. Some states offer higher levels of protection of assets that are in an 401k/workplace type of accounts as opposed to IRAs. I'd consider that before converting to an IRA.
If it was me, and the protections in my state were not any different, I would do a rollover IRA to Vanguard if that's where it made sense to go to, and do the workplace 401k as you propose with the new plan and (rolling it after leaving the workplace).