I filed my 1099's and 1096 but I forgot one vendor. How would i proceed with
this?

you need to send out amended or corrected 1096 Annual Transmittal letter including not only all the previous 1099-Misc already reported to the IRS but also include the 1099-Misc that was missing. You may write in red the word Amended 1096 at the top of Form 1096 and then include all the 1099-Misc that are to be submitted to the IRS.