Advantage cannot be granted by Dispatch. - I assumed that it could, and that caused a number of misunderstandings. Vassals, in basic mode, are not required or even Able to offer Advantage to their liege, unless this is specified in your domain secrets.
The basic vassal treaty that is default, is just the agreement to pay gold and rp, no more.
I've used method 2 with my own vassals, simply developing a more explicit treaty that clearly defines that I gain full holding advantage. (that means I'm allowed to use their holdings, as were they mine)
And in at least one of my other diplomacy treaties, I've allowed the other part to contact my vassals directly, offering a treaty that I have the right to approve/veto.
And finally, I have a treaty where I am to get the vassals ratification on behalf on the third party...
So, its really flexible, as I can imagine it should be. But the main rule is that diplomacy is required to create the basis.
I've used Decree to add "friends" in some treaties, in which case a "friend" is recognised once both treaty partners has spent a Decree action to ratify the "friend" to not receive Hardness.