You have to put your function in the python path (I don't know where it is on your system).
For testing it, let's assume you named the file "myfunction.py" :
Code:
import vapoursynth as vs
import myfunction as mf
core=vs.get_core()
clip=mysourceclip
clip=BrightDfttestMod(clip)
clip.set_output()
Or, instead of using the "import myfunction as mf", you can copy/paste your function in the script. Which is good for testing with vsedit in my opinion.
ps : you forgot the import "vapoursynth as vs" before "core = vs.get_core()"