Turbochargers in general shouldn't really make boost at idle conditions. If it did, the exhaust plumbing would be overly restrictive and the turbocharger sizing would be much too small to support higher power levels.
When the vanes are fully opened, the exhaust is allowed to flow freely to the turbine. Thus, as soon as extra power is generated by the engine, in any amount, the exhaust will increase in heat and volume. Boost will then be generated when the turbine speeds up. At low power levels, it may seem that the time delay is excessive. Usually referred to as 'turbo lag', it is one of the big factors in judging turbocharged systems, particularly compared to supercharged systems.
The vanes used as control of boost as opposed to using a wastegate has the advantage of retaining some back pressure in the exhaust manifold to increase response as well as being able to control the boost level with a much better level of granularity. To achieve the same level of control with a wastegate would require an array of small wastegates, which would be a large disadvantage both in terms of installation (size, weight) as well as cost. Computer control of the vanes is both cost effective and more efficient.
Sorry to wander afield of the original question, but maybe it will help someone else.
Cheers,
PH