I don't find any way to manually enter vitals into the Health app. It appears you need to use the third party monitoring apps that are listed the Vitals page. Some of these require and Apple Watch. If there is a way to manually enter the data I haven't found it yet.
It you click on Vitals > Blood Pressure > then the plus in the upper right hand corner you can add manual data. Similar to how you can add Body Measurements (BMI, height, weight, etc.)