Hi Lal,
I first learned about these 2 words from a bhante (based on his own course material). So, I’m not sure about the original source. Then, I googled them and found some info.
“There is no vedayita sukha bliss in Nibbána – there is only Santisukha.”
“The state of Cessation of Perception and Feeling is a ‘pleasure, not being felt’ (avedayita-sukha).”