Some aircraft (like the Lockheed XFV and Convair XFY) take off and land pointing straight up, and transition to level flight in the air. How hard would this be to add?

I might have just realized it appears you're misinterpreting the question. There's a difference between the XFV and a Harrier. One is still pointed forwards but the thrust is directed down with rotating nozzles (the Harrier), the other is actually pointed straight up with the nose to the sky, and takes off like a space shuttle (the XFV).

So what you're saying is that you want rocket engines for MTS?

Technically that's already possible. You'd just need to get the plane to orient itself in the upward direction upon placement and have a strong enough engine to overcome gravity. The latter is easy, the former not so much, as your center of gravity is going to make you try to tilt downwards on takeoff and the plane won't be able to land without stalling itself. I'd much rather do helicopters than this, but if someone makes a way for this to work, I won't stop them from implementing it in the mod.

No. Let me throw some links at you. This is the kind of plane I'm talking about.
https://en.m.wikipedia.org/wiki/Convair_XFY_Pogo

And this one. https://en.m.wikipedia.org/wiki/Lockheed_XFV

Alright. I'm going to close this for now. Feel free to re-open it if you have nothing better to do for the mod.

This would require dynamic thrust. While I should be able to add it, it will take quite a while to design the physics system for that, especially with planes having issues already with slowly falling to the ground due to a faulty physics system. Helicopters are far more likely to be the short and medium term solutions to vertical takeoff.