We have some custom buttons on the Sales Order form to create PO's from it ( Drop Ship PO etc). I'm trying to create a WF to set a custom field to the same value as the Vendor field after field edit (which is sourced from SO) but I'm having trouble getting it to work. I'm not sure if it's the scripted creation of the PO, and how that might act differently then a normally created PO. Any help would be greatly appreciated? I apologize in advance if none of this makes sense.

so you have a Go To Record action on the SO WF, yes? i take it that custom field is not an option here?

There is no WF on the SO, it is done by scripting. I have created a different PO WF with Set Field Value action but for some reason it isn't populating the desired field. I've tried multiple different Trigger On options such as after field sourcing/ after field edit/ entry/ after record sumbit etc. Nothing seems to be allowing me to set my custom field yet.

Have you checked your logs?what do they say ??