generating saif file
saif file - contains toggle counts and time information like how much time a signal was in 1 state(T1), 0 state(T0) , x state (TX). Also in backward saif file you can have timing information, arc information just like .lib (read saif manual).
vcd file - contains value changes of a signal. i.e. at what times signals changes their values. saif does not contains this information. it contains cumulative information of vcd.
you can convert a vcd file into saif file using Synopsys vcd2saif binary and see the effect.
Effectively vcd file is a superset of saif file. Any application which needs time stamps of individual value changes must have to use vcd e.g. time based power estimation or if you have a vcd file which is simulated between 0 to 10ns then you can do power analysis for any time range between 0 to 10ns e.g. power for 2-5, 3-7...
but for saif you can not do you have to regenerate saif for these time intervals.