Without realizing it I guess I've been following your viv journal on DB already! LOL
Wonderful tank - I love seeing growing in photos.
Yes you can cut the main vine once you start getting basals, just be aware that one of the basals will start to elongate to replace it but you will have many months before it's also too long and needs to be trimmed as well.
As far as rooting the cut section of N. ampullaria vine you can certainly try, of all the Neps I've grown ampullaria seems most reticent about not rooting from a vine cutting. I have no idea why that would possibly be but the basals can be divided up and grown on to new plants no problem. It may have simply been the old clone I had that didn't root from stem cutting, I've got 3 new N. amp forms (harlequin, orange/red and speckled) who may not behave the way my old green form did.