have somewhat a naive question, or three rather
1. did POW which rewarded miners with STEEM ended ?
Yes that was only during the pure PoW phase.
2. if true to the above, can current POW that rewards miners with VEST be converted to STEEM ?
Yes but conversions are only 1/104 per week. It is probably best to mine to increase your Steam power (formerly known as VESTS) if you intend to use the system. But you can mine, convert, and sell if you really want to.