Possibly a certificate of employment, stating you are currently employed and as part of your duties remote work in Thailand and other areas is authorized.
There is no income requirements, just the 500K funds requirement. Present your contract and certificate of employment stating your remote worker authorization and single payslip if you wish.
I Believe all the Thai embassies in America are using online system, your application will be forwarded to the consulate with jurisdiction over your location.
Is one a new passport and the other expiring soon? Sounds tricky as you'll need to show you're in Vietnam by showing your visa/entry stamp, and the passport numbers won't match. If you have a plausible reason you could write a note to explain.