Registration - using another ID

[Chamber]

I'd like to remove the vBulletin registration altogether, and replace it with a bespoke system.

However, I'd still like to use the vB generated cookie, and other board functionality.

The process I envisage is thus:

Bespoke registration page filled in.
Most data saved in new custom table.
Insert ID (for cross referencing) and essential data passed to vB database - > standard registration confirmation email sent.

What I'd really like to know is:

a: What is the "essential" info that vB needs. I'm thinking username, password and email address - anything else?

b: Since I'm handling the registration through a direct connection with the database, how might I envoke the email confirmation bit?

c: Any obvious flaws in the plan?

Many tia.