Hollywood studios have been juggling the opening dates of at least 18 films, each hoping to get the jump on competitors in the crowded holiday movie season. According to Exhibitor Relations, a trade group that monitors release dates, the star-studded Bonfire of the Vanities was set to open in November, then Dec. 14, then the 19th, and finally the 21st. Though Warner exec Rob Friedman says the only date the studio announced was Dec. 21 and that other dates were outsiders' speculation, he admits everyone is jockeying for position. ''We wait and watch to see what the competition's doing,'' he says. Orion, for instance, has changed the opening of Mermaids with Cher and Winona Ryder from Nov. 16 to Dec. 21 to Dec. 14. So frantic has the game become that Exhibitor Relations last month had to junk its printed schedule twice. ''We've never had anything like this,'' says president John Krier. ''We've never had so many potential hits in the market at one time.''