I'd say that vendor lock-in is unrelated; any platform can have it regardless of the openness of the platform.
Jailbreaking is not part of the lock-in. It's a way of doing more stuff with the hardware than the original software allowed you to do. The fact that the term has 'jail' in it doesn't make the users more locked in than they already were, they were free to enter and leave without jailbreaks.
Tangentially, I used to jailbreak my iphone because it offered me useful extra features that I couldn't otherwise get (e.g. a SSH client back when Apple didn't have such apps). But I haven't felt the need to jailbreak iOS for years now, because just about everything I could want to do with the system is already possible.
Finally, 'jailbreaking' is effectively another word for 'security hole'. If my iOS device can be jailbroken, it's actually a worry to me, not a benefit.
Google's chromebooks do offer a better alternative to this, whereby you can flush out ChromeOS by making a physical change to your device (in my case, disassembling the laptop and turning a particular screw). But it still can be a security risk; if there was some physical means of replacing iOS on a phone with alternative software, you could never trust that a secondhand iPhone hadn't been maliciously tweaked to preload malware.