Vista backup full vs incremental

Hi,

I have been using Vista backup (Vista Ultimate) for a couple months now.

The first month all was well. It did a full backup, then incrementals each
night. This is to an external 750 gig USB drive.

Around day 35 backup failed because it over-filled the target drive. I was
surprised on two counts. First, it didn't do a full backup at the one-month
mark. Second, it didn't detect ahead of time that the target drive was 99%
full.

To resolve this issue I manually deleted the previous backup (dangerous, but
I'm stuck at this point right?). And then I used the wizard to start a
backup - and it did a full backup.

The next night it did an "incremental" backup that copied everything.
Another full backup, but in the same sub-folder as though it were
incremental.

It has been doing a "full incremental" backup every night since. And of
course I run out of target drive space in 2-3 nights at this rate.

I can not figure out how to get backup to start doing real incrementals
again. Ideas?

Nor do I see an easy way to schedule a full backup every two weeks (which
would help solve my fill-up-the-target-drive issue).

I really like the simplicity of Vista backup. But I'm beginning to really
dislike the lack of control as compared to ntbackup.

Any help or insight is appreciated!

Rocky