I’m very impressed with the MBB design. It makes sense that the solid front triangle gets power from the pedals to the wheel most efficiently, but I would have guessed that a rider couldn’t get power to the pedals efficiently with such a design. Cruzbike's records, rider testimonials, and videos of the bikes flowing down the road in a straight line prove otherwise. I would order a Vendetta sight unseen, but am hesitant about the fixed seat angle. One of the things I like about DFs is that they allow you to alter your position as appropriate to the situation. I probably ride 90% of the time on the hoods with my elbows barely flexed… cruising in a pack chatting with the rider beside me, climbing a hill, sitting up on a downhill to cool off, picking my way along a crowded sidewalk, eating, etc. But when the need for speed arises, it is easy get down on the drops, bend my elbows, and get my torso near horizontal (or it was when I was younger). It would be nice to be able to sit up, or lay back, without too much complication on a recumbent. I would suggest hinging the seat back, making it spring loaded, and using a quick release to lock in a more upright, or more laid back position? You might to use carbon fiber for hinge and spring.
I would assume it would be best not to have the seat bottom move with the seat back. I built a prototype recumbent back in 1983 and struggled finding a seat bottom I liked. I eventually came up with what you see in the picture below. It may not be a good recumbent marketing idea, but it works well. There is much less weight on your sit-bones than on a DF, but having the nose of the saddle between your legs steadies the frame just as with DF.