[Remnant campaigns seem to get priority over override campaigns]

do you mean you the zone keep showing the remnant campaign, no matter an override campaign is linked ?

 

[Using Vast Code On Advertiser]

you can use fc.php to deliver the xml for vast.

reference link : http://www.vimp.com/en/web/faq-tips-tricks/items/how-to-use-openx-for-video-ad-delivery.html

[Underdeliverance When Using 'contract' Campaigns]

I have the same problem.

now, i need keep monitoring the delivery and manully switch the campaign to "exclusive" to take all the impression