Some OEM's high side drive the VTC solenoid (Honda and Subaru) but there is no specific need to, just what they chose to do. Some of the early Vtec hondas (B serias and possibly D series) they had a single wire VTEC solenoid where one end of the coil is grounded through the body so the ecu needs to switch the positive side. But still a half bridge is not needed, just a high side drive or more commonly just a relay.
I suspect where the confusion is coming from is just the loose use of the term "half bridge". Motec for example call their aux outputs "half bridge outputs" just because they are capable of high side or low side drive, but you dont use them as a half bridge to drive a solenoid. They only work in half bridge mode if set up as a stepper motor drive or in a pair for E-throttle (but when 2 half bridges are paired like this it is usually called a full bridge).
On the Xtreme Aux 5-8 are half bridge outputs, but we dont call them that as the only time they actually work as a half bridge is when set up as a 4 wire stepper. For all other functions such as VTC or idle valves they only function as low side or high side drives.
A half bridge is generally an output that drive both high side and low side, but that term can be used a bit loosely in some circles. It is not needed for VTC, just a conventional low side drive aux is all that is needed. Connect one wire to any spare aux output (Aux 1-10) and the other side to ign switched +12V supply.
