Never tried the VTDI, but had the VT Bass V2 about 10 years ago.
The VT has control over the mids, while the FX91 doesn't. The inherent character (based on memory) is different. The FX91 wrt treble/highs, is more metallic clank, while the VT is more fizzy sizzle.
I remember the lows having a very "plump" character on the VT, while the lows are more filled out and thicker on the FX91.
While the gain range on both can get out of hand real quick, the FX91 has different "sweet spots" that are easier to dial in.
I know the VTDI has a blend, which my VT Bass did not.
Full range clean blends can be useful at maintaining your lows, but with certain circuits/settings can have this "layered" effect, that can be off-putting.
The low-passed clean on the FX91 is incredibly well implemented, sounding very natural, without the layered effect I mentioned earlier.
If there's anything else you're curious about, don't hesitate to ask.
Thanks!
Your description of your VT Bass V2 is very close to how I fell about the VTBDI and the blend control is not THAT big a deal.
How would you describe the overdrive on the DOD?
Is it similar to a tube amp breaking up and is it touch sensitive?
I like the VT but I do sometimes think the edge of the distortion is a bit fizzy when it could have more bark.