Comfort Noise Insertion in VAD algorithms I am in a intention of Porting Voice Activity Detection algorithm in real time hardware,so i was successful in classifying speech and silence/noise but now the problem is replacement of silence frames with comfort noise(preferebly any stationary noise)

so can any one suggest any opensource CNG algorithms or how a comfort noise can be of any type(pink,white etc) 

to do this usually we use a rand() functions but where as these type of functions are not available with real time DSP so how to port such type of Algorithms?
You may want to try a two-pronged approach.  First, have a standard pre-packaged CN to inject (like jan and Jason R suggest) at the start of the conversation/exchange.  

As the conversation progresses, gather low-volume data, perhaps as part of your noise reduction algorithm I assume you already have, by sampling the lowest volume signal coming in between VAD end and start points. Compare your low-volume samples to your standard CN, and replace (or convolve to simply dilute your synthetic CN) if it matches some threshold or characteristic you established. All you need is some rand and normalization functions, and perhaps FFT, all of which are readily available in C (and assembly).