The rack itself is rated by Racktime at 44lbs. This is of course subject to how it is supported on and mounted to the bIke. In the case of the Vado, Specialized published cargo load figures for earlier year models that limited the rear rack, as mounted, to a 30lb cargo. I'm not aware of any changes to the rack mounting or support so I've personally assumed that the 30lb limit still applies. Still a fair amount of load, but less than the LBS quoted.
You can find the Vado weight limits by searching for "Specialized 2018 Vado 4". Go to the pull down "Manual Downloads" menu under "Technical Specs" and select "2018 Rider/Bike Weight Limits and Terrain Conditions". I have uploaded this spec to a different thread.