Allocations by Vendor?

Anyone setup an allocation schedule that sources based on vendor before?

We have some vendors whose bills will always be allocated to different departments.  SaaS software is a good example.  Let's say we get a monthly bill for 10k from Supreme SaaS Solutions.  Every month we want this to hit a single account, Software & License Expense, but we want to allocate it among 3 departments in a 60, 30, 10 (%) split.

We can't just base the source on the Account as there are other transactions hitting the same account that we don't want to allocate in this way.  I tried putting the Vendor Name (e.g. Supreme SaaS Solutions) in the Name field on the source tab, but it's not pulling anything in.  I'm thinking maybe because the Vendor Name doesn't post to line expense level on the bill, only the header level for AP.