Hey Scott, another dirt/preamp thread, we meet again! Ha! lol
I currently own both the VXL and VT and they're both awesome, it just depends on what you want and how much $ you have to spend.
The VT is pretty awesome for a scooped Ampeg dirt with lots of color, the Hartke would be more for "clean dirt" if that makes sense... less color on the dirt sound and no huge mids scoop. The DI and Shape circuit have also proven to be quite handy. You can definitely get dirt with it... Not "fuzz dialed to 11 melt your face off" dirt, but it's got a really good rich sounding crunch. And it can get absolutely huge, too. Like, damage your speakers huge. In the last run through on this clip, I had to turn the level down to two
for unity volume. Hartke VXL rough clip
If the amount of dirt is simply all you're looking for, the VT does have more gain available. The VT is also a bit more aggressive and you can dial in a more cutting sound, if that's your thing. If you haven't seen this video, it's what finally pushed me over the edge to buy a VT. When I have the Tech21 on my board, I also use it as an always on, and actually my settings are pretty close to what he's got it set at at 2:01, maybe with just a little less gain. Tech21 VT Bass
Also, the VXL does have one weird little quirk... the level knob functions all the time, regardless of the pedal being engaged or not. I've never found that to be an issue though. My VXL is currently an "always on" in my home rig (which I also use for recording) and my VT is currently on the sideline.
Hope that helps.