My Downtube FS experience was that it was not good in crowded transit situations. Getting it up the Embarcadero staircases in SF during morning rush (wall to wall people and packed trains) was 100% no fun. It's an extrememly awkward fold for carrying in crowds or for long distances, and fussing with a bungie to hold it together seemed silly compared to the easy fold of the Dahon (FWIW, people here say that the Downtube Mini is better).
I think you've already got a really good bike for the price, keep modding the Vitesse. The Big Apples are excellent on my Speed P8, that's a great idea too.