Est. 1998 — 27 years of woodworking knowledge
Multiples
I was told (by Al Crandal, a former GM Tool Room Supervisor) that the 1.5 figure is a machinist's standard. It is also effected by the tool holder and the placement of the boring bar support. The Kobra uses a "goal post" support of 1" hardened steel rod that is locked to the banjo and can be set very close to the opening.
I'll see if I can post a pic or two of the whole rig.
Re: Multiples
I sure don't mind vibration 8-10" away from a small opening. Who is going to evaluate the surface quality that deeply into the vessel?
No-o-o Body
Except, maybe, another turner 
Only thing about vibration is it interferes with the "feel" of the cutter as I'm hollowing. Get too much of a harmonic going and it will feel uncomfortable like something's about to cut loose.
Thorns
I've always wanted to do a hollow vessel with thorns around the opening, and title it, "Go ahead and check the interior". Only turners would get the joke!
Re: Thorns
Observation
Having walked a few "craft" shows where turned jars were offered, I noticed shoppers interacting with stuff. The men would almost always turn the piece over to look at the bottom, but it was the women who tended to try to feel the inside surface.
I suspect this behavior may be a genetic leftover from our specie's time as hunter/gathers where the women (gathers) were more likely to try to find something edible inside a shell. This behavior is still exhibited by other primates today, even to the point of using a tool of sorts (stick or straw) to get at what's inside.