Any point in hanging on to vassals? (or making marches)

I like having one or two Marches when in later parts of the game. The reason for that is that marches provide you with more troops than you could yourself support off of that very land (it's different culture group and/or different religion). Also, it's nice to have marches with strong NIs (Brandenburg as a march is a brutal thing).

I like vassals to have around as vassal states munching some wrong religion+culture land, saves an insane amount of admin+dip. I guess now that they are less docile that might no longer be such a great idea...

Vassals and Marches both effectively give you more troops and manpower than directly owning the land would. Vassals have their own force limit; and then contribute some to yours as well [More with Influence Ideas]. Taking my common Etheopia start for example; directly owning Kaffa might give me 2~3 more Force Limit. Having them as a Vassal gives me personally maybe 1/2 force limit; but they themselves have 5+; and an effective second pool of manpower. I keep Kaffa as a vassal until their armies are literally no longer any benefit at all.

Also sometimes they're useful to keep if they're taking military ideas while you focus on other ideas. Or as mentioned before; great NI's which help their troops be more effective than yours.

Feeding Vassals can save you Paper Mana for later.

Of course; a Vassal has the downsides of less direct tax income; no direct control over their armies; and the chance of them rebelling or being freed more easily if a war goes badly, and eating a Diplo slot.

No reason to keep vassals if you aren't role playing. Make a few of them, feed some provinces, annex all of them. Make a few more, feed, annex. Repeat.

That's the most efficient way to expand your nation in your continent, if you aren't an OPM.

keeping vassals has several "bonuses" which are really just alternative ways to play.

A vassal will contribute more men to a war than annexing the territory. THis is because they get a base increase of men. They also have thier own manpower pool.
A vassal can make claims, be fed provinces, and convert religions. THey also help deal with pesky unnaccepted cultures and nationalism.
Enemies seem less likely to attack a vassal serving as a buffer. This may be a bias on my part, but I often make a buffer Hansa between scandinavia and austria when I play, because It seems to deter the white plague. And the french later on.

Marches exarcebate these "bonuses" with the price of making it much hard to annex later on. I am currently working on a sardinia Piedmont playthrough that will focus on using marches and vassals rather than direct conquest, and will tell you how effective it is.

I have always had great use of marches, but it depends on the nation.

Brandburg/Prussia is a favorite of mine, they have excellent ideas and can provide you a nice aggressive army to back your own forces up with.

In my most recent playthrough as Ottomans, one of the better I have had, I have had Crimea and Serbia as marches - almost since the beginning of the game. Crimea has been really helpful, Serbia mostly went and got killed to begin with, but after feeding them a bit of Hungary and all of Bosnia/Croatia they have consistently had a 16 stack army helping my sieges and hunting down enemy stragglers.

I'm a very casual player compared to many others on the board so maybe it just fits a more laid back playing style to keep Marches for a while. It suits my game well too, because i generally only expand west when an opportunity arises. I am focusing on getting to India/Malacca/Australia this playthrough and creating a Turkish colonial power