It’s midseason replacement time at the networks as ABC premiered “Vital Signs” on Thursday night and NBC launched “Crisis Center” Friday night, with both placing third in their timeslots.Time-period shifts for current shows are also underway, although early results are not setting the world on fire. NBC, which as usual won Thursday night, premiered the freshman sitcom “Suddenly Susan” in the 8:30 p.m. slot while “Single Guy” takes a break before moving to Wednesdays later this month. “Susan” averaged a 14.9 rating and 24 share, according to Nielsen, its lowest rating to date. In the 9:30 p.m. slot following “Seinfeld,” where the show launched this season, it averaged a 18.2/28. On the plus side, “Suddenly Susan” retained 89% of the “Friends” audience, only 1% off from “The Single Guy’s” average retention. On a down note for NBC, “The Naked Truth” (15.4/24) hit a new season low and posted the lowest numbers NBC has had in that hot time slot since “Madman of the People” was airing in 1994. ABC’s reality medical show “Vital Signs” posted a 6.9 rating and 11 share in the web’s 9-10 p.m. slot on Thursday. While hardly great numbers, it beat the 5.7/9 that “Murder One” was averaging in that slot. For the night, ABC was third with a 6.5/11. CBS finished second with a 9.3/15 while Fox, which broadcast the “NAACP Image Awards” was fourth with a 5.7/9. On Friday night, NBC premiered the drama “Crisis Center” in the 10-11 p.m. slot held by “Homicide.” It posted an 8.1/14 share in the 36 metered markets, finishing third behind ABC’s newsmag “20/20″ and a special edition of “JAG” on CBS as the Eye web gave “Nash Bridges” the night off. On Saturday night, ABC launched “Dangerous Minds” in the 8-9 p.m. slot, where it averaged a 5.6/10. For the night, CBS won in the metered markets, followed by Fox, which was airing a figure skating special. ABC just nipped NBC for third place. National ratings for Friday and Saturday will be released this week.