I can't find the length of chain for the Vampire Chain. I've schoured the Rapax Rift looking for it and to no avail. I'mm also looking for Plumed Serpents. Can some one tell me where the nest?

One of the prisons--might be Rafe's--has the chain.
The mountain and southeast wildernesses are flying with the plumed serpents.

the chain can be hard to see. use your mini-map in the bottom left, you'll see the white dot. it's behind him towards the back of the cell - you'll have to squeeze up close to reach it.

and forget about the plumed serpent feathers for that armor. the armor isn't that good AND plumed serpents are tied to your highest charactes level (which I've found needs to be really high).

I haven't been attacked by this particular type of flying serpent until about the last 10% of my games - usually at AP.

The mountain wildernesse secret dungeon has 3 plumned serpents in it.

I sold the cahin before seeing Ferro. Is there any other way to get it?

Sorry to say, but there is only one chain. Unless you were training someone in mace & flail don't worry about it. There are many other weapons that are superior to the vampire chain.

The Mountain Wilderness secret dungeon, like the other two, changes monsters based on your lev. I saw no plumed serpents this time around, but there are certain monsters you always seem to meet.

If i remember right aren't Plumed Serpents stronger than Flying snakes but weaker than acid wings?
So whether you see them or not is dependent on your current level?
ie. Flying Snake>Plumed Serpent>Acid Wing

In my experience, many of the monsters are always the same (the ones that wait for you behind certain doors). The random monsters are different, but only the graveyard dungeon has a significat number of them in my experience.

Maybe you didn't enter the room with them?
There is a room in this dungeon with 3 teleporters behind doors. IIRC the middle one is an exit, the right one takes you to the room with many teleporters, and the left one to the room with 3 plumned serpents and 3 anthrazite glems, and from there to the teleporter room.