Measure antenna S11 next to another source
The other sources will be uncorrelated with your miniVNA signal. They may contribute noise to the measurement, but will not offset the mean result.
Any conductors within a few wavelengths of the antenna will affect the S11 measurement, so make sure its environment is the same as it will have for the final deployment. You can check the antenna's sensitivity to the environment by moving hands or conductors around near the antenna while you make S11 measurements.