Hmm, I noticed that the userids in `userfield` and `usertextfield` match with those in `user`. However, all those members that registered using my registration form do not have a row in those two tables... what should I do about that?

Also, im guessing that I just have to increment each of those tables by a row so that a new row with the userid of the user registering is made?

Create the missing rows :)

When you insert a new user into table user you will gat an insert id (=userid).
Use this to fill the other tables.

crap, they're a whole load of ones i'd have to add then... knowing that `userfield` and `usertextfield` are empty, what problems would users be facing? what happens if they try to modify the stuff thats there through the forum?

the userid number is auto_increment, so just creating a new row will create the new id for all the other tables too right (this is on registering a new user).

They can't modify their signature, buddy-/ignorelist, pm-folders, subscription folders, search preferences and custom profile fields like Location, Interests, Occupation and Biography.

Yes. Wherever there is a userid, it is the ID for this user (in table user).

You can fix these problems through ACP / Import & Maintenance / Update Counters by clicking Fix Broken User Profiles.

This could more problems if also new members have been added using standard vB functions.

I'm fixing the broken user profiles, and then modifying the registration script to add a new userid in the `userfield` and `usertextfield` tables. That should solve the problems.