In mid-May, Saint Vincent Hospital in Worcester announced it had opened a total of 102 positions to permanent replacements for union nurses who have been on strike since March 8. But Marlena Pellegrino, a registered nurse who is one of the leaders of the strike, said she’s skeptical many nurses are stepping up to fill the positions. Many in the heavily unionized profession refuse to cross picket lines. But, even beyond that, Pellegrino said, filling positions at the hospital was a challenge even before the strike began.