Are you talking about adding effects pedals (aka stompboxes)? I've used pedals such as overdrive, EQ and chorus with the Vox AD30VT with no problems. I've even used a Digitech RP-80 Multi-effects/Amp Modeller. The only issue I have is that some of the modelled amp tones didn't sound that great with pedals, especially with the RP-80 (which I attributed to over-digitizing of the signal with a modelling effect going into a modelling amp).
The VFS-2 is a handy footswitch as it allows hands free switching between CH1 and CH2 (preset channels on the AD30VT) as well as effects ON/OFF or switching to manual mode (in effect giving you 3 channels to use).
The VFS-2 does not go into the signal between you amp and the guitar, it plugs into it's own jack in the back of the amp. Any generic latching footswitch with a 1/4" TRS connector should work.
DISCLAIMER: I don't know much if anything about the technical workings inside electronics, so take take this with a grain of salt . Consult the manufactuer if you have any concerns.