How do I deal with vapour barrier for central vac inlets in exterior walls?
&quot;I am thinking to just form a small pocket around this whole thing, and then tape up the holes where the wires go through, and tape around the 2&quot; suction line.&quot;

Seems like you answered your own question.  I like that you're being fastidious about the vapor barrier, but I have some questions for you.
How new is the house?
How comprehensive is the existing vapor barrier and what does it consist of?  Typically here in coastal California we don't emphasize the vapor barrier as an integral part of the construction process because the indoor/outdoor temperature and humidity differential is not so great.  This disqualifies me as an advisor on this aspect of the project but I'm interested in learning more.
Is the garage at all a conditioned space?
From Wikipedia:

An interior vapor retarder is useful in heating-dominated climates while an exterior vapor retarder is useful in cooling-dominated climates. In most climates it is often better to have a vapor-open building assembly, meaning that walls and roofs should be designed to dry:[6] either to the inside, the outside, or both, so the ventilation of water vapor should be taken into consideration. A vapor barrier on the warm side of the envelope must be combined with a venting path on the cold side of the insulation. This is because no vapor barrier is perfect, and because water may get into the structure, typically from rain. In general, the better the vapor barrier and the drier the conditions, the less venting is required.[7]
In areas below foundation level (or, subgrade areas), particularly those formed in concrete, vapor retarder placement can be problematic, as moisture infiltration from capillary action can exceed water vapor movement outward through framed and insulated walls.
A slab-on-grade or basement floor should be poured over a cross-laminated polyethylene vapor barrier over 4 inches (10 cm) of granular fill to prevent wicking of moisture from the ground and radon gas incursion.
Inside a steel building, water vapor will condense whenever it comes into contact with a surface that is below the dew point temperature. Visible condensation on windowpanes and purlins that results in dripping can be somewhat mitigated with ventilation; however insulation is the preferred method of condensation prevention.

Based on the information you've provided it seems that your house varies from cooling to heating needs depending on the season.  This implies to me that the integrity of the vapor barrier is of minimal importance.
It has been my experience that condensation problems primarily happen where the air-flow in uninsulated locations is retarded by improper insulation installation.  A small (&lt;1 ft. sq.) area where the vapor barrier is nonexistent because of fixture installation is unlikely to be a problem.  E.g. everywhere you have an electrical box.