Help understanding forum user integration w/ my website

[travisp]

I have an existing website with a small number of beta users with an already created user authentication and registration system.

I've decided that I'd like there to be a forum that shares users with the existing website. Although I'd like a login on one website to login to the other, the most important thing is that users don't have to register twice in order to encourage participation.

I've read a large number of posts on here about integrating websites with vBulletin, and have come away a little uncertain about the best way to proceed.

What I wanted to do - Keep existing system:
Maintain the existing website's user tables, registration, and authentication system and sync this information with the vBulletin database when changed (it's unclear to me if this is technically a "bridge", but I think so).

Pros:

1. Already have registration and authentication working how I want it.
2. Seems that this still might require less modification of both my existing website and vBulletin than the second option below.

Cons:

1. It seems relatively easy to do this one-way, but it seems harder to sync back from vBulletin to my website, for example if the user is on the vBulletin forum and changes a password or email address (it seems that it would require either a lot of hooks or redirecting the user from the forum to my application for anything that would change this information)
2. There's a possibility that databases will get out of sync if I make mistakes, or that I will fail to ensure that the requirements of user information match the requirements of vBulletin (e.g. field lengths, etc.).

Second option: use vBulletin's system exclusively
Scrap my registration and authentication system and use vBulletin's (probably by including vBulletin files and calling the API from what I can tell).

Pros:

1. May eventually result in less maintenance because I won't need to maintain my own system.
2. May actually be more secure because the vBulletin system has been more battle-tested.
3. Gets rid of the possibility of the databases going out of sync

Cons:

1. More closely tied to the vBulletin system and license.
2. Appears harder to separate the forum from the existing website (e.g. hosting them on different servers to protect resources). Would the license even allow me to host the forum on one server and still include global.php on the website server for example?
3. Possibly extra overhead of calling vBulletin functions from my website to handle authentication and access control.
4. Custom plugins may still be necessary to get the vBulletin registration and authentication system working the way I want them to.
5. The feeling of annoyance at not getting to use the code I already wrote and tested

Does anyone have any thoughts on this? I feel that an integrated forum with my website would be a great benefit to the users, but I want to be sure that I do it right, so I appreciate anything that will help me better understand the integration options available to me.