What is the purpose of ferrite or inductor on the voltage input of FTDI chip?
My plan is to ignore it, but I am afraid it might be important for the
  stability of this chip


Ignore at your peril: -



If you google around for USB interface circuits that use this chip, you'll find the appropriate device to use for the inductor. I've build one myself (and did the same) but, for the life of me I just cannot remember what I fitted other than that I went on the hunt for what other people had used and got an answer. 

It's a bit crap that the data sheet isn't more forthcoming on this part of course. OK, here's a start: -



Located in this document. They are 600 ohm ferrite beads. I expect if you dig a bit more you'll find the part number. Use them.

You can find the part number in this document. It's an 0603 size ferrite bead so not hard to fit and, there is never a good reason to ignore them.