Packaging is the problem, both width and length. W/ a transaxle such as from a Pantera etc. the engine is too far forward, it would be necessary to move the driver position so far up that your face would be against the windshield (you're already pretty close in a stock Vega), then where does the steering wheel go? Only alternative would be to stretch the wheelbase rearward, which wouldn't do much for the attractiveness. With a transverse setup things are too wide for the narrow H-body, there's not enough you could shorten the axles to not have it looking funny, i.e. rr tires sticking too far out compared to the front. Toronado transaxles have been used, same problem...axles have to be so short the suspension doesn't work any more.
The Toro's been done a couple times I've seen. Was behind one on the freeway and he appeared to be at the threshold of control...at 55. Although it looked good at the show we were both going to.
Not saying it couldn't be done, but all car designs are a compromise somewhere. Mid-engine in a Vega is strong on "neat-O", weak on practicality.