I dunno, in the short term it's true that making friends with them like this is certainly less likely to create conflict, but long term they are sure to eventually turn on us and if they are absorbed that's a lot of conflict now but eventually it'll probably simmer down. I think that if you're thinking long term then obviously absorbing them is more secure (especially when you consider we don't get any new possible hostile neighbors from doing so, unlike if we say, absorbed a southerner that wouldn't really give us security because we'd have lots of new potentially hostile neighbors, if we absorb these guys that just means we get a bigger border with someone we're already pretty much next too.)
What I think is that there are always enemies. If we owned the whole world Haspen could still roll out Martians. Project into the long term and absorbing all of Parsia doesn't change our need for preparedness by one iota. So yeah, I'm working on the nearer term where I can see actual differences in which scenario presents fewer challenges to our army during the period they rebuild from war losses.
We can just annex all of Parsia without a problem, they can't oppose us, we occupy their capital and most of Parsia already. By annexing them we'll gain control of their gemstone mines, which will be nice boon to our economy.
The current offer is terrible compared with what we'd get if we just annexed them, they're offering us 10 years of tribute from a gemstone mine that we'd gain if we annexed them. In the case that the parsians will revolt after being conquered, our army will easily handle any revolts.
There is a school of thought that expansion on the map is the paramount measure of success. I can respect that and
partially subscribe, it has a lot of truth including your point about the gemstones. Yet spamhistory suggests that greediness and overextension breeds revolts, extra expenses, and in the worst cases downfall. Permanent gemstones are a minor advantage when I expect next year's surplus to be +4 or better anyhow, and who knows what they yield ten years from now?
I'm not even certain that we would keep the mine in question: if it is in vassal territory game rules may force us to yield it to vassal, aka NPC, control.
Ten (or twenty, the duration of the non-aggression pact) years from now our army should be ready to go again if we want more shinies that badly. And we won't be fighting vassals who have had access to our superior tech.