The person at Vanguard is largely mistaken. Although the account can be moved to your own by reportable distribution and rollover, it is permitted and preferable to move it by nonreportable trustee-to-trustee transfer as you did with the inherited IRA held at a different firm. Moving it by distribution and rollover involves the one-rollover-per-12-months limitation on IRA rollovers while trustee-to-trustee transfers of IRAs do not.
(There is a special rule regarding catch-up of RMDs in certain situations for surviving spouses subject to the 10-year rule, but the 10-year rule does not apply with regard to decedents dying before 2020, so the special RMD catch-up rule does not apply.)
If Vanguard refuses to move the inherited IRA to your own IRA at Vanguard by nonreportable trustee-to-trustee transfer, you might consider moving the account by nonreportable trustee-to-trustee transfer to an inherited IRA at another firm that is more accommodating (which Vanguard should certainly know is permissible), then doing the trustee-to-trustee transfer to your own IRA there.
Note that you should perhaps not be receiving any Form 5498 from the other firm, certainly not one reporting any rollover since you did no rollover. Unless there was a deposit into the IRA other than a trustee-to-trustee transfer, an IRA custodian generally just provides you with a year-end statement and supplies an actual Form 5498 only to the IRS.
