To avoid noise from the external system I tried to isolate the internal completely. So I added DC-DC isolated converter with signal isolation. I have attached the picture. Still when reading data and communicating with VFD it receives data (sometimes garbage) and the PC cursor shows unusual behavior which gets data from the FTDI(FT232R) connected to it.
There could be a few reasons here. The disturbance may even be radiated rather than conducted on A and B from your circuit's point of view.
As a simple place to start; your drawing doesn't seem to show a shared ground connection from the isolated side of the RS485 to the remote terminal. Without that you are in the realms of 'might work' and you may be exceeding the common-mode performance of your transceiver. An alternative (worse) approach to a shared isolated ground reference is to have 'leak' resistors from A and B to your local signal ground, fail-safe biasing resistors can create the same result too.
Isolation is not a cure for all noise problems, as Cursorkeys says the noise you see might very well be radiated.
I think you'll have to try and see what works for you.
This resource helped me when I had to deal with issues similar to yours: https://support.fccps.cz/download/adv/frr/RS485/RS485.html