The maximum reading on concrete for vinyl installations should not exceed 3 on the top scale on the meter. Moisture in wood subfloors should be measured with a wood moisture meter. Read flooring manufacturers moisture standards. A moisture meter can also be used to measure moisture in concrete. Follow the instructions carefully.
Moisture meters measure wood and concrete differently.
The Accelerated Plastic Film test is a good indicator test for concrete moisture. Tape 2' x 2' poly film squares with duct tape to the concrete subfloor in several areas throughout floor. Lift the squares after 24-72 hours. If moisture condensation is present on the film or if the concrete has darkened this indicates moisture is present. Create a starting line and secure a straight edge or backer board at this line.