I was in my attic doing some wiring and came across a 2" vent, I
accidentally touched it and it moved..."moved" as in slid down for
about two inches.

The 2" cast iron vent is original. I hired a plumber to do some work
on the bathroom below, and because the original vent line was in the
way of a new wall opening, he created a new 2" PVC vent line about 3
feet off the original cast iron vent line, then in the attic, he cut
the original cast iron pipe about 6 inches below the roof, then the 2"
PVC line turns towards the cast iron vent, and connected to it using a
no-hub coupling right above the turned up elbow.

So my bath is now all PVC, until it goes up to the vent and there is a
short piece about 6 to 8". That in itself is not a big deal...however
the fact that when I touched it it slid down is a problem. Once it
slid down, the up turn elbow is now lower than the other end, meaning
the almost horizontal run of that pipe in the attic is now sloping in
the wrong direction.

Not a problem, I just screwed in a piece of 2x4 between two ceiling
joists and some pipe straps and now it is sloping correctly.

However it makes me wonder, the fact that the pipe is sliding, I am
thinking it should have flashings and sealant against the pipe right?
does it mean the roof seal is broken? I have not gotten up on the
roof yet, I am a bit paranoid to climb up there. What is the proper
way to fix this? The second question is, since the pipe is sliding, I
assume if I were to loosen up the no hub coupling I should be able to
pull out the entire contraption on the roof? if so should I just glue
in a piece of PVC all the way instead of using a short 8" cast iron
pipe?