As others have indicated, the MiL has to apply for the B-visa (B2) for the visit. Your wife is not a US citizen yet, so the IR-visa (IR5) cannot be used to sponsor her mother for immigration at this time.
This may apply to big cities only (HCM/Hanoi/etc), but my MIL was able to get the police certificate at the promised 2-weeks turnaround based upon their new "streamlining" process. No agency or grease needed....