You don't need to replace the feedback resistor. Simply
wire in the pot so that it acts as a variable resistor that
goes between the speaker jack "tip" connection terminal
and the existing feedback resistor. Use the pot's middle
terminal and the left terminal (as seen from the back of
the pot) to make the connections.

A good rule-of-thumb is to use a pot whose value is
somewhere around five to ten times the value of the
fixed feedback resistor. At the maximum clockwise
setting, the pot will drastically reduce the amount of
negative feedback being applied.

In larger Fender amps, the best place to put this pot is
in the existing chassis hole for the extension speaker
jack. I believe the VibroChamp has only a single RCA
jack to connect the speaker, with no extension speaker
jack. You might have trouble finding a good place to
put the pot without drilling a hole somewhere (which is
something I would discourage!).

You might want to save this mod for another amp
anyway. Being a single-ended class A amp, the VC
probably doesn't use all that much neg. feedback to
begin with, so the pot would likely not have a very
noticeable effect. 


And if I were you, I would use a large cap before the
pot (wired as a variable resistor, like Jim said); I have
seen people use 1uF 50V ones there wihout any
problems. That way the pot will not see any
DC,otherwise it's going to be very scratchy sounding.

Gil 

I'm OK with the blocking cap but be very careful in
value and in the placement of the pot/cap combo
because you can introduce a very nasty high frequency
oscillation doing this when you turn the pot. 
ON Fender amps I can't see any reason in using
anything over a 10Ka to 25Ka pot.
They really don't need or react well to be loosend up
too much.
I use a 10Ka in the mods I've done for folks.

Bruce 

One last question: When connecting the wires to the
pot, do I need to ground the unused terminal to the
body of the pot? 
Thanks,
DanR 

No, just leave 'em alone. 

i did this mod to my 5f2a princeton, single ended with a
7868 tube. i wired a 100k pot in series with the 22k
resistor. i see a drastic difference! i love this mod, with
the pot all the way down it is stock and has much less
volume and attack, with i all the way up the clean
sounds so good it is unreal! when it goes into clipping it
gets nasty though, so i back it down a bit. it is a very
easy mod to do with exellent results! on your champ, if
it has a 22k fb resistor and you dont wanna drill, try a
56k resistor in there.