Messages doesn't entirely cut you off from people who don't bring money to Apple, and it doesn't require another app to communicate with them. It will handle and integrate it all in the same interface with iMessage, seamlessly switching in the background to make that possible. Whatsapp, OTOH, plainly refuses to let you interact with anyone who prefers not to accept an intrusive TOS and bring in (ad) money to Facebook.
I agree that both apps have different approaches and both have pros and cons, but I don't support the suggestion that Whatsapp, or any similar app for that matter, is any less "vendor lock-in" than Messages.
I'm unsure of what do you mean when you say you see the same "vendor lock-in" in both cases.