Hi Marky,
If you specify clearing criteria for GR/IR account in OB74. Then after posting MIRO, you have to run F.13 for the account. Then only it clears automatically. However, at the time of MIRO posting, it will only post document not clear the balance automatically in the t.code. The vendor balance will get clear once you make payment by F-58/F110/F-53.
Regards,
Mukthar