"Why there is soo many medvac vacancies open a the present ??? the minimuns are too high, or there is a lack of helo pilots in the U.S???"
There is a problem attracting the experience needed at the compensation offered.
See this for more:
compensation
"How good is the working conditions offered by those companies?
EX job security, benefits, etc...."
There's no comprehensive answer to that question. Generally speaking, companies hate to lose a productive pilot and strive to retain them, unless it costs money...
Yes, EMS pilots are on call for mandatory overtime. My employer has agreed to contractual restrictions that aren't too onerous. I haven't been "called in" in a decade.
If your base produces and facilities lease endures, your job is generally secure unless you screw the pooch somehow. Bases do move and close when those factors aren't there, and the industry is always in a competitive flux.
I haven't observed the inter-professional strife between medical and aviation that is often remarked on. I've worked with some pretty "challenging personalities". I know people who know people who know of medical running pilots off.
I haven't done the offshore (Gulf of Mexico) thing in 16 years. The money was better, but you live out of a bag and never sleep at home while on-shift.