I did try to use the legs as a mounting platform for the vise. A traditional face vise has to be mounted to the "underside" of a bench, thus you have to create a false underside to mount one on the MFT/3. I thought that the weight (which is significant for a good vise) and lateral, inward "pull" force on the legs might strengthen and stabilize them. Unfortunately, I found that the opposite was true.
The vise platform, mounted to the legs, made the legs bounce when the vise was used and created a distinct vibration in the whole table. In addition, the weight of the platform, and the bouncing, seemed to put a strain on the legs where they are leveraged against the underside of the table. I became concerned that I might be doing irrevocable damage to the legs.
I've found that the legs of the MFT/3 are less stable than I need, and aren't mobile. That's why I built a cart. I've seen numerous excellent examples, on this site, of shop carts for the MFT/3 ranging from very elaborate to simple and utilitarian (like mine). If you are dependent on the portability of the MFT/3 and its legs, then I wouldn't suggest trying to install a vise at all. Otherwise, I obviously recommend a cart. Good luck!