First of all, i have signed the treaty with Dorgania and it did not cause any ruckus with the other two nations, i think it would only allow you to make a few trades with Dorgania on the overview map and increase Central relationship with Dorgania. And no, there is a no treaty signing with Amagal and Begus yet.
Secondly, i think best equipments in for Vanguards cannot be bought or crafted, they are randomly found through battles (unsure) or extra bandit battles (the "unusal bandit spotted" will apppear on the turn they spawn and they'll only be there for 1 turn)
(in my case, i have found a perfect war horse from an extra bandit battle)
You can try opening chests in those Vanguard battles with thief guy's pick locking ability too.