I also think your vet is an idiot! Most puppy series of shots should be complete at 12 weeks but at 9 weeks they should have sufficient antibodies to allow them to be in controlled social environments. There is no window of opportunity more important than the 8 - 16 week period of a newborn pup to be socialized to it's environment. The 7 - 8 week period is invaluable for learning to socialize with it's pack. That is why I hate to hear of dogs being taken from their litters before 8 weeks.

Please Note: The Jack Russell Terrier is now officially called a Parson Russell Terrier as of about a year ago or so.

I have seen several PRT's put down because of this aggressive trait. On the other hand, I have seen PRT's that are the sweetest, most loving dogs ever in the hands of a persistant owner. You need to read and learn more about this trait and what can be done to help the dog before it is too late. The AKC has even published this as a recognized fault in the dog because it is becoming more wide spread through poorly planned breedings.

I strongly encourage you to read more about the breed on the AKC's website. http://www.akc.org/breeds/parson_rus...rier/index.cfm
They also have many listings for PRT clubs. Contact one of them to try to get some help.

In the mean time, be firm in what is acceptable behavior and never reward behavior that is not. Find something the dog loves and use it as a teaching tool. JRT's usually love to play ball. If the dog acts out when it goes for walks, consider walking it with a halti collar on. This is a muzzle restraint system that is dog friendly.