That still doesn't help if the OPAMP is running from the same supply as the collector of the emitter follower. You either need a boosted supply, or a topology similar to that used in a LDO regulator, e.g. a PNP pass transistor driven by an error amp, with the regulator's reference replaced by a fixed proportion of your target Vcc.
Also, if you've got to support a 5V target, its not exceptional to encounter the case where your target Vcc is up to 10% high while USB Vbus is as low as 4.75V. That can result in target logic '1' level being over 0.7V higher than your devices, which is enough to cause issues for CMOS inputs that aren't over-voltage tolerant.
I therefore suggest that you boost to say 6.5V to allow enough headroom for a simple OPAMP + emitter follower to regulate your buffer Vcc to match a target Vcc of up to 5.5V. If its only going to be powering a few buffers, there's probably no need to improve efficiency disabling the boost for target Vcc below 3.5V.