I can't help with your ROI--I'm very much a newbie to vipassana--but I can address a couple of your points.

Do I get to control my personal demeanor to improve my relationships and get better outcomes when dealing with people?

This is why I'm doing it. I've been having a specific problem dealing with certain kinds of emotional situations, and since I started meditating it's been much easier for me to let my initial negative reaction to those situations pass, and then choose how I would prefer to deal with them. So it's not about strengthening the rational part of my brain, it's about clearing an obstacle that was keeping me from using it.

Additionally, during most of my meditation sessions, I've felt very comfortable and in control. It's too early to say whether that will translate to greater confidence in the rest of my life, but I have had similar experiences before (confidence in one area -> confidence in others), so I'd be a little surprised if it didn't.

it just turned out to be fun (which I don't expect meditation to be)

I've found it fun. It's interesting observing physical sensations (or the lack thereof) which I'm unaccustomed to, and some of them are entertaining. I described some of those experiences in the vipassana open thread. There's also jhana, which Will_Newsome describes here:

Incredibly intense feeling of bliss, compassion, and piece. I involuntarily laughed at loud about five times. I think there must have been some kind of feedback loop going on here. I felt clearheaded.

Incredibly intense body high. My whole body was quivering, including especially my eyelids. It was a numbness-like feeling, though perhaps different in that if felt like quivering. It could be that my perception of the feeling had changed.

...

Previously I'd heard that meditation could lead to feelings of profound bliss, compassion, and even a sort of very strong physical body high. I'd mostly discounted such reports on the grounds that 1) I've done some drugs and didn't expect the effects to be as strong as e.g. cannabis, and 2) it didn't seem clear how just focusing on your breath could cause significant physiological changes of the sort necessary to have such strong effects. After experiencing jhana, I can say I was wrong.

Maybe you can bliss out easier on drugs, but meditation is free. ; )