I use a weighted training vest as well, along with ankle weights, wrist weights, and weighted shorts. I always warm up first with a full body stretch, and a couple thousand jump ropes. I've never had any problem with spine compression, but I've never used that much weight on the vest. My vest is about 20 lb. (can go up to 40). I bought my vest because it was the only vest I've seen that really stays on; I can easily jog or jump rope without it bouncing or moving at all (here's the site
http://www.strength-systems.com/feat...ght_vests.php). The vests I tried before this one were always loose and bounced every which way, so if that's what your's does then I could see that causing some spine trouble. I agree that you should consult a physician/specialist. You also might want to lower the weight; 40 lb. for a jog might be overdoing it. It sounds fishy, because I've been using mine for nearly two years now and have never had any trouble, but if your doctor tells you otherwise let me know!
What kind of vest do you have?