Dear Saumendra

 $$$ "Depending the Required CFM in the rooms " $$$ .

Then Room Temperature , Each Room Air Pressure , Type of Usage in Rooms, RA CFM  and FA Mixing CFM According to Design Requirements that have  Can be decided .  

I think that they are the same thing.

A VFD refer to AC drives only while VSD refer to either AC or DC drives. VFDs vary the speed of an AC motor by varying the frequency of the motor. VFDs referring to DC motors vary the speed by varying the voltage to the motor.

VFD should prefer for high load while for small load VSD should be preferred.