"I imagine I can somehow isolate the buzzing and delete it"
Yes.
You can remove a constant background noise like hum or hiss from an audio track. In my situation I had an audio recording marred by a Super-8 film projector mechanical noise in the background, and I was able to completely remove the sound of the projector.
I'll condense a few steps here...Export your audio from your captured VHS into a wave file. Download and install the freeware open source audio editor Audacity. Import your audio into Audacity. Select a section of the audio where the buzzing (and nothing else) occurs. You are creating a noise profile. Then select Noise Removal and Audacity will use the noise profile to identify the bad noise and go through your audio file and remove the noise. Then bring that file back into PowerDirector.
http://wiki.audacityteam.org/wiki/Noise_Removal
You can get more information by Googling "using audacity to remove background noise"
Audacity is not the only audio editor capable of noise profile and noise removal, but Audacity is popular, and free.