UPDATE: Vuforia 5.5.9 has been released and supports Unity 5.3.2 and 5.3.3; it also automatically disables BITCODE in the Xcode project generated by Unity, so that it will compile without issues:

https://developer.vuforia.com/library/articles/Release_Notes/Vuforia-SDK-Release-Notes

https://developer.vuforia.com/downloads/sdk

Also, make sure to check the Unity Migration Guide before upgrading to Vuforia 5.5:

https://developer.vuforia.com/library/articles/Solution/Migrating-Unity-Projects-to-Vuforia-5-5

OK, thanks. I caught the sticky and some other post regarding the downgrade, and I have read your suggestion on other forums — but I didn't realize that "enabling bitcode" meant setting the bool to NO. Counterintuitive, but worx, thx.

This has been addressed already in a few other Forum threads;

you simply need to go to your Xcode project Build Settings and setthe option "ENABLE_BITCODE" to NO.

The reason for this is that, starting with 5.3.1 patch p1, Unity set the default setting to YES (while it was set by default to NO in all previous versions, including 5.3.1f1).

Other than this, you should be able to build and run Vuforia 5.0.10 with  5.3.1p3 without issues.