hello i just started playing this game a month ago and as of now i am currently at the end game(trying my luck to get the skillcrystal called dispel) then i just remember that i have this bond with dorgania which leads to this question "if i accept the friendship treaty with dorgania will it automatically declare a war with begus and amagal?" i still haven't decided to accept it since i might regret the decision later on and it seems i can't form with the same treaty with begus or amagal. Is it not yet available to form a treaty with other 2 countries on the current update? or am i missing some missions/events in order to trigger it?

also how can i get the unicorn mount and other powerful equipments in overview map?

(ps. sorry if my is english bad it is not my 1st language.)

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.

Same with me, you better not answer non agression pact with Dorgania yet. Don't accept or refuse. Wait until developer unlock option to declare war with other countries, or you might regret your decision. We can answer it later.