Man, this is a great game and mod 
I have a few questions,
How does one pay off fleets so they don’t attack you?
Can one ask for peace/ceasefire?
Can I declare war so my allies vote to start?
Thanks
Vengeance fleets can not be bribed, they'll chase you no matter what. Your best bet if you can't defeat them is either juke them with transverse jump or by entering a system and exiting from a different jump point or lead them into a system owned by a faction hostile to them (they won't try to avoid it).
Invasion fleets also can't be bribed, you're best bet of stopping one if you can't defeat it is raise your relations with the faction before they reach your colony or invade the planet from which they will be launched before they appear. Only AI inspections, saturation bombardment fleets (if you built a colony in another faction's territory) and expedition fleets can be bribed in the intel screen.
To make peace you simply need to raise your relations with the faction you're hostile to either via missions, AI core bribes, prisoner exchange or agents. You, the player, cannot ask for a ceasefire. However if a faction's war weariness goes too high from fighting enemies (go to intel, dipl.profile, search for the faction you want info on and you should see their war weariness, their alignment (which determines if they'd be willing to go on an alliance with you and their traits that increase/decrease their relations with you depending on circumstances (AI usage, owning more markets than them, having a free port... etc) ).
When a faction's war weariness gets too high they will eventually offer their enemies (including you) a ceasefire which you can accept or reject (note: if you're in an alliance and the other factions that are with you refuse to make peace then only yours won't be hostile to them while the other will still wage war on them)
And finally if you're in an alliance then simply lowering your relations with whatever faction you want to be at war with will start a vote once you become hostile with them.
