Vanguard cannot find my account--seriously

I have an interesting problem with an old Vanguard account that was created in 1993. To get to the point and spare all the details, I forgot about the account until now (2025). I know, this falls on me. However, in 1998, I left an employer and they transferred money in to my Vanguard account. From that old employer I have the date of the transfer (remember this is pre internet), the amount, the Vanguard account number it went to, the employer plan and the address the check was sent to (which I confirmed is a valid address for Vanguard). I have called them numerous times and spoke to about a dozen people and they say they cannot find any record of me having an account--of course they searched by SSN, address, DOB etc.--still nothing. The only thing they tell me is they need more information. I ask them what else they need and they just say more information and it goes round and round. How in the world can they have no record of the account? My next step is to request the cancelled check from the old employer in hopes that it has some useful information and then write a letter (sick of calling and getting nowhere) explaining the situation. What else can I do? There has to be a record of this somewhere in their system.