Decoupling caps wouldn;t affect the cathode voltage at V4, or is that not what you meant?
First question is this: is this an actual problem or is it just something you didn't expect, but noticed the amp doing? In other words, you may have some channel crosstalk, but when you are playing can it be heard then?
How do you even hear bleed into the normal channel when you are playing the Vib channel? There is no post control on the vib channel. I would expect the sound through teh vib channel to be 10-20 times louder than any crosstalk from the other channel.
V1b and V2b have their cathodes wired together, and that is certainly a place for interaction, especially if the shared cathode bypass cap is dried up.
If you suspect a decoupling cap, then don't guess, find out. The cap in question would be the sideways one beside V4b. CLip another one in parallel with it and see if it does the trick.
Clearly if you pull V1, then the normal channel can;t amplify anything, V1 is the whole channel.
Crosstalk can come from many things. Decoupling is just one of them. SIgnals radiate around inside the chassis. Regardless of problem or no problem, isolate the symptom. SO the normal channel picks up the other channel. I will just assume you have a way to listen to just that channel. So listen to the crosstalk sound. Does the normal channel volume control affect its loudness? DO the normal channel tone controls change the tone of it? Does the normal channel bright switch make the crosstalk brighter? And is the grounding contact on BOTH inpout jacks of the normal channel working? Are both jack tip contacts grounded? Those tests will tell you if V1a or V1b is picking it up. Poor wording, while it is possible I don't mean the tube itself picking it up, I mean the tube STAGE picking it up, V1a or V1b. Using a clip wire, what happens if you ground the grid of V1a? And the grid of V1b?