Which of these are redirects?
 

For example, I created my own external registration page for vB. Out of these hooks, would I use to make a plugin that redirects the user to a page of my choice after he/she signs up?

register_addmember_complete
register_form_complete
register_signup

Also which one of these would I use to redirect a user to a page of my choice after he/she activates their account by clicking on the link in their email?

register_activate_form
register_activate_process
register_activate_start

Speaking of the activation process, is it possible to create an external page for it?

Look at register_addmember_complete, and the code below it. You will find $vbulletin->url.

Okay, thanks. What about the activation?

Did you look at the hooks you posted yourself? register_activate_process

I was asking because I had tried something before & it turned out to be the wrong hook. I think I've got it all now. I'll come back if there's something else.