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If you apply for a premium service from VFS to submit your UK visa application in the USA, your biometrics will be taken at the VFS visa application centre (VAC). You do not need to visit a USCIS ASC separately. This applies to all services from Bronze level upward. See also What happens at the premium application centre
I've heard of people being able to walk in at the USCIS ASC, but if I were in a hurry I wouldn't count on being able to do it.