Vapor intrusion will likely come up in the "reasonable steps" analysis, as one considers what reasonable steps are necessary to prevent or limit exposure to previously released hazardous substances.  VI may also be addressed in a land use restriction or institutional control, particularly with sites with vapor barriers or prohibitions on groundlevel residential use.  In either case, it will be identified in Step 2, and addressed in Step 3 & 4 of the Guide.

VI is an interesting case for a couple reasons.  First, VI may be the chief exposure pathway of concern for many commercial/industrial sites, particularly where groundwater is not used and soil contact is not likely.  Second, mitigation for VI may be more cost-effective than investigation, especially for new construction. As a result, it may be cost-effective for a user may require a vapor barrier as part of constructing a new building, even without 100% assurance that VI is an issue at the site.  Such precautionary measures are mentioned in the Guide as possible reasonable steps, which is another example of the flexibility in the Guide. As with other continuing obligations decisions, site-specific conditions may or may not limit the available options.  

As a final note, for owners that address VI from off-site releases, there may be legal options to recoup the costs as well.  Of course, that's assuming you can identify the source and the source is solvent, among other considerations.