I know this is weild, but my experience of vaulters are they are extremely passive-aggressive, even beyond cravers and UE: in my latest game as riftborn, vaulters blocked my very first outpost with their starting scout ship (instead of using it for scouting), settled right next to my capital and attacked me with pirate marks (the turn my outpost's just done) and disguised ships, doing all this from the other end of the constellation.  I have ecountered this more than once whenever they share the same starting constellation with me.  My horatio start with neighboring cravers was not even this difficult.

So, could a diplomatic option that tells the rival blocking the player's outpost to leave be added to an update sometime? 

BTW, in case anyone wants to take shadenfreude from my misfortune: my home system's Regulus, with Vega was my first outpost, Kais being first vaulter colony outside of their capital; my second outpost, Triime, was finished before my first one...

Yeah, that'd be great to have. Otherwise you are rather shoehorned into a direct war approach to deal with the issue. 

This is definately an issue.

   I'm sure that normally you get a "remove nearby outposts" contextual diplomatic demand in a situation like that. The problem with the Vaulters is that they are one of the instant colonizer-style factions, where there's no outpost stage in their colonization sequence and thus do not trigger the "remove outpost" demand. They should maybe add a "please don't colonize nearby systems" request for diplomacy with insta-colonizer factions. Or on second thought add that for diplomacy with all factions, as it would be nice to have a chance to prevent a situation like shown in your screenshot when dealing with outpost-colonizers and insta-colonizers alike.

   In the meantime, another option other than combat in situations like this could be to purchase the offending colony in diplomacy. My experience is that trading things for colonies tends to be much more affordable in Endless Space 2 compared to say the Civilization series. This is anecdotal of course: I've traded reasonable amounts of tech and/or resources for colonies in this game, where the biggest hit I felt was to my influence reserves.

Good. If there's no previous idea about this, make one. PM me if you need some votes ;)

I'm onboard with this 100%, to be honest the game needs a lot more new diplomatic requests for different scenarios, I might make my own thread in a similar vein as this one soon.

Agreed.  That and possibly a few more diplomatic/espionage options to liven up the relationships with other factions as well (e.g. versatile non-aggression pacts including guarantor status, protection of trade routes, coalition against a powerful empire, option to accept refugees, incite rebellions, assassination attempt on a rival leader/hero... perhaps a few of these might already be within the scope of the upcoming expansion pack)

I normally nuke any jackass that blocks my outpost, but yea, taking a few planets for an outpost is an overreaction on my part. So I'd like this.