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. gil

I'm OK with the blocking cap but be very carefull 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? 

No, just leave 'em alone. 

Hi all, I have seen on some amps with this type of mod,
is to parallel a cap either a disc or a coupling cap with
the feed back resistor to take out some of the harshness
one might get when turning this effect up, it filters out
some of the nasty highs, but you have to experiment
with cap values to get the desired results to taste. I have
seen .001-.02uf depending on how much highs you
want rounded off. So maybe the cap can be soldered to
the end lugs of the pot? I had put a 100k resistor with a
.0047 uf cap in parallel in the feedback loop of my
friends Blues Deville and when cranking that thing it got
fatter and not harsh at all. What works for some may
not for others though, try it!
Good luck,