The purpose of this study is to provide open-label vicriviroc to HIV treatment-experienced subjects who successfully completed 48 weeks of treatment on ACTG protocol A5211 (or who responded favorably to treatment but discontinued participation due to viral tropism shifts), and subjects who screened for ACTG A5211 and met all inclusion and exclusion criteria, but were unable to enroll due to protocol closure. Vicriviroc will be administered in combination with an optimized background antiviral regimen, chosen by the investigator based on results of genotype and phenotype testing, the subject's history of prior antiretroviral drug use, and drug toxicity. OBT must include at least 3 antiviral drugs (not including vicriviroc), 1 of which is a ritonavir-boosted protease inhibitor.