This code is a bit confusing architecture-wise. getCustomPropertyFloat() hints the reader that the framework for the properties is uniform and can be reused by other assistants, but the tool still manually check for the assistant name and property name. I guess the code should either be simplified to dynamic_cast'ing to VanishingPointAssistant and just requesting this property directly via explicit function call, or a complicated system for "abstract properties" should be done, like in KisSliderBasedPaintOpProperty.

I guess, for this specific case, it would be easier to just use dynamic_cast. In contrast to these manual checks it delegates compiler to check stuff for us :)

Please use KisDomUtils for reading and writing property data. Using QString::number() and QString::toFloat() will not work in languages that have comma as a decimal separator (e.g. German or Russian).

You can either use KisDomUtils::toString()/toDouble() or use more hi-level routines like KisDomUtils::saveValue()/loadValue(). Since the rest of the code doesn't use save/loadValue(), I guess you should just use toString()/toDouble().