I know that with a lot of FX I have an FX track and send my other tracks to it.
However, for the purpose of getting rid of background noise should I apply this VST separately to each track or can I do a send with that too?
In general I like to remove noise independently in each track first, then process everything else.
Because all these tracks could have different noises or not a constant noise all the time in the arrangement.
Doing ReaFIR in each track gives you more control.