Most plugins - if coded correctly - work at any sample rate.
Example:
Take a delay. If you want a one-second delay, you just have to read out the current sample rate (any VST plugin can do this) and delay the signal by this value.
So if you're at 44.1 KhZ, delay 44100 samples, at 96 KhZ delay 96000 samples. That's easy because the result is always a delay of one second. A half second is just half of the value and so on. It works perfectly at any sample rate.
So when a developer says that his plugins work only at a certain sample rate, he is often just too lazy to read it out from the host and use it as a variable
Oversampling plugins do as follows:
At first the signal is upsampled (x2 oder x4 from the current sample rate).
Then it is processed (Reverb, comp, or any DSP).
At the end the sample rate is brought down to the original rate.
That means that this one plugin works as if you would set your whole project to a higher sample rate. These plugins often sound better (less aliaising) but use more CPU power.
I like plugins where you can switch oversampling on or off. Then you can listen for differences and decide if you need it or not.
Some plugins aslo have an automatic feature for this. They keep the sample rate while you are mixing, but process an oversampled signal when you are (offline) rendering.
I don't know if up/downsampling would work in a chain, like
1. upsample
2. any VST plugin
3. downsample
because I don't know how Reaper handles signals between plugins.
Maybe someone knows this.
XoechZ