You have a couple of options here. Consider that by plugging your bass directly into the Focusrite, you're not really using the bass amp for what it was designed for (amplifying an instrument signal), and instead having it do what would be better served by a set of full-range monitor speakers. You'd be effectively using the Focusrite as a DI box and your bass amp as a monitor. The Focusrite is perfectly good at that, but you might want to get some more benefit from your bass amp.
One option would be using the effects send and return jacks on the amp to create an "effects loop" using your Focusrite and VST processors. You'd just need regular TS 1/4 inch patch cables. Your signal chain would go from your bass to the regular input on your amp, then connect the amp's effects send to a line level input on the Focusrite, then from a mono line level output of the Focusrite to the effects return on your amp. (You can always plug a balanced jack into an unbalanced one or use a TS cable instead of TRS -- it just means the whole connection is unbalanced, so don't do it over long distances.)
I'm an amateur at reading circuit diagrams, but I believe that the effects return on your amp will create a bypass of the existing signal, so you get the benefit of the preamp inside your Ampeg, you can do whatever you want to your sound on your PC, and have that come out of the amp speaker. You still get to use all of the controls on the amp, they just affect the signal before it goes into your effects processing.