The shingles vaccine is a live attenuated virus, which means the shell of the virus is alive, but the nasty inside bits are removed. Therefore you are NOT contagious at any time after taking the vaccine unless you have an active breakout of shingles.
The shingles vaccine is the chicken pox vaccine at 7 times the child's dosage. A booster shot for adults isn't a bad idea.
In the phase 4 trials, the vaccine was over 90% effective which is wonderful compared to the alternative. While it would be nice if it worked 100% of the time, but alas this isn't perfection. Those in the 90% now live with far less pain and suffering due to the vaccine. The shot is well worth the cost.
