  Dark halo substructure may reveal itself through secondary, small-scale
gravitational lensing effects on light sources that are macrolensed by a
foreground galaxy. Here, we explore the prospects of using Very Long Baseline
Interferometry (VLBI) observations of multiply-imaged quasar jets to search for
submilliarcsecond-scale image distortions produced by various forms of dark
substructures in the 1e3-1e8 Msolar mass range. We present lensing simulations
relevant for the angular resolutions attainable with the existing European VLBI
Network (EVN), the global VLBI array, and an upcoming observing mode in which
the Atacama Large Millimeter Array (ALMA) is connected to the global VLBI
array. While observations of this type would not be sensitive to standard cold
dark matter subhalos, they can be used to detect more compact forms of halo
substructure predicted in alternative structure formation scenarios. By mapping
~5 strongly lensed systems, it should be possible to detect or robustly rule
out primordial black holes in the 1e3-1e6 Msolar mass range if they constitute
>1% percent of the dark matter in these lenses. Ultracompact minihalos are
harder to detect using this technique, but 1e6-1e8 Msolar ultracompact
minihalos could in principle be detected if they constitute >10% of the dark
matter.
