I've not been able to get a successful bulk scan run with 1.8/2.0 of Video thumbnails very likely due to the time/memory limit a php script is allowed to run (in constrained hosting environments).
Is it possible for the plugin to run staggered scans in batches of, say, 10 at a time?
Addendum: During a bulk scan, is it possible for the plugin to replace placeholder featured images with its updated video thumbnail ?
An open idea for other users: If the author cannot (understandably) attend to this request, there may be hope in a very long winded approach. It might be worth simulating a dev environment by trying mirroring your wordpress to a LAMP/WAMP PC and running the implementation, and then synchronizing the database back with the online hosted version.