First of all, it may be that this is not a vRA problem.
The vRA AD integration actually works, which means that the computer object is created in the correct OU.
The problem now is that the object is created 1:1 like the server name (e.g. srv001), but the server is joined as SRV001 and to the default OU when joined via the Customization Specification.
vRA 7 you used to be able to do some pretty cool stuff out of the box with VM placement into a designated OU using custom properties. Unfortunately, this concept isn't in vRA 8 at this time. I imagine that to place an object in the OU you want, you may need to write and run a vRO workflow to pre-create the object in AD at one of the vRA 8 extensibly lifecycle states (perhaps compute.allocation.pre or compute.provision.pre) and then when your customization spec runs, it will join the domain