@Keeper of the Keys wrote: so just to be clear distributed trunking would not work between the vsf stack and a third switch?
Exactly. That's because the entire VSF (made of your two Aruba 5400R zl2 switches) is seen just as a simple single switch from the peers connected to...for the very exact reason it forms a single logical entity (the "single data plane"). A normal port aggregation (usually with LACP) is enough to connect ports on both VSF members with a peer switch (northbound or southbound).
@Keeper of the Keys wrote: It says in the guide that distributed trunking is incompatible with VSF but if I have 2 5400 linked using VSF and I have a trunk using ports on both 5400 isn't that a distributed trunk? Or because VSF changes the 2 switches into "one" switch it's a "normal" trunk?
VSF lets you to avoid Distributed Trunking for the reason explained above...and the port aggregation (Port Trunk) you configure on the VSF (say you aggregate port 1/A1 with port 2/A1 using LACP with the usual trunk 1/A1,2/A1 trk1 lacp CLI command, as example) is exactly the same you will use on the peer switch (as example, trunk 51,52 trk1 lacp)...exactly because the peer switch is going to see one switch entity (the VSF) and not two separated ones as would happen in a distributed trunking scenario. In other terms DT and VSF technology are mutually exclusive (or, better, VSF was engineered to avoid having to deal with DT).