You have to remember, when you are measuring a resistor as a two (or four) terminal device, you are measuring the sum total of what is going on inside the resistor, all of the operating variables are in play and what you are seeing is the sum total. If you want to measure any one given parameter, it must be done in isolation, which in some instances can be difficult, otherwise you are still just measuring interactions between parameters. For instance, you cannot accurately measure humidity effects without holding all the other parameters constant, otherwise you will get an erroneous measurement. Since few if any have done controlled humidity measurements correctly on resistors, I cannot consider any of the statements about humidity effects on any resistors as valid here.
For instance, the interaction of VC and TCR; the TCR could be positive and the VC negative, in which case you'll see the net sum difference which by chance could be quite small, they could also be of the same polarity in which case you would see a much larger difference, which on the outside would appear as a larger TCR. In the case of multiple resistors operating in a circuit, such as you noted, you will see the net sum total of all of the parameters interacting with each other including environmental. Again, depending on the individual components, you may see a very small change in output or a larger change in output, it may not be linear either, it could be 'bumpy' and rest assured it will not be the same for each circuit with different components.
This is one of the reasons that we have stated that testing the components in the actual circuit under actual operating conditions will give you a much better idea of actual performance vs. taking a bunch of measurements on a DVM (or resistor bridge for that matter) individually. That will tell you very little about how they will perform as a group in a circuit.
Just to set the record straight again, PWW resistors do not have VC nor do they have 1/f noise. BMF appear to have lower 1/f noise than the other film/foil types, this is based on published papers which I have not verified myself for the BMF resistors. Like most resistor parameters, VC has a ± limit and any one individual resistor can be anywhere within that range, just like TCR or tolerance, it is unlikely that the VC parameter has an actual zero value though due to the physical effect that generate it. PWW resistors consistently have the lowest inherent noise level possible with the BMF appearing to be very close behind.