System Information
Operating system: Windows-10-10.0.18362 64 Bits
Graphics card: Quadro P6000/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 436.02

Blender Version
Broken: version: 2.80 (sub 75), branch: master, commit date: 2019-07-29 14:47, hash: rBf6cb5f54494e
Worked: (optional)

Short description of error
The Title says it all. If an object has a Particle Instance Modifier on it, it does not generate a Vector Map nor does it Motion Blur.

Exact steps for others to reproduce the error

1. Create a Particle System on the default cube (I used a plane for clarity in my demo scene).
2. Create a new object (I used a cone)
3. Bake the the Particle System to a cache (for ease of scrubbing the timeline)
4. On the new object, cone, add a Particle Instance Modifier
5. Select the emitting object (cube) and choose the particle system (default is ParticleSettings for me)
6. Any combination of modifier settings is fine; for clarity I checked on Regular and Size and then adjusted the size to be smaller in the Render > Scale section of the ParticleSettings system.
7. Switch to Cycles in Render Settings
8. Turn on Motion Blur in Render Settings
9. Turn on Vector in Layer passes
10. Scrub to a point where you see particles in your timeline moving
11. Turn on Use Nodes in Compositor
12. Render image
13. In Compositor you can drag Vector to your Composite Image input. You'll note no image appears, nor does any Motion Blur appear.

Attached is a demo scene, with a few Forces to move the system around for clarity's sake.