Here are a few suggestions:
First, I recommend the use of a rooting hormone, even with
the easier species, such as your N.ventricosa, to ensure
success. There is a product called "Dip 'n' Grow", which works
quite well. It is a liquid, which seems to ensure better
penetration into the cutting than most powders. Alternatively,
you could also use one of several gel products available, which
are also good, such as Olivia's, or Doc's Clone Gel.
Secondly, keep in mind that the issue of most immediate
importance in rooting a cutting is the availability of moisture.
So, be sure that humidity is high; if not, your cutting will
dessicate long before it is able to grow roots. However,
be sure that you can maintain the appropriate nocturnal
temperature drop for this species; N. ventricosa is an easy
plant to root, but if it is stressed, rooting may not occur
readily. Also, when you make your cutting, be sure to
cut the root end of your vine segment(s) at a very
oblique (slanted) angle. This serves to increase the area
of the cut surface and therefore provides better hydration.
Finally, regarding your question about upper vs. lower pitchers
produced from an upper growth cutting, I can tell you that
if your cutting is producing upper pitchers, pitchers produced
further down the vine, even after it is cut, will be uppers.
However, when a new, basal node on your cutting breaks,
pitchers produced from that point will usually be lowers.
Hope that this helps you. I certainly don't know all there
is to know about rooting Nepenthes, but I have rooted
literally hundreds of cuttings over the years, so this
advice is based upon experience!