Heyo, self-proclaimed Piapro expert here!
For CUL specifically, she'll work in any of the VST versions of Piapro. The process is just installing and activating as a normal Vocaloid VB, and then in Piapro, she'll be grayed out and require a license. Upon trying to select her VB, they'll give you a button to get a code from the Sonicwire website that you copy/paste and will be able to use her full time.
In genereal, all V2 and V3 VBs will work in the VST version of Piapro. V4s will only work if you have the V4X version of Piapro (included with Luka, Rin/Len, and Miku V4X only). Piapro Standalone (included with Miku V4C) does not support any additional VBs. In other words, if you obtained Piapro via Katio, Meiko, or Miku V3, you can use V3s but not V4s.
V2 VBs must be imported to V3/V4 via the Vocaloid Import Tool prior to being accessible in Piapro. If you have a used copy of a V2 VB, you might not be able to get an import code from the companies needed, but V2 import codes are not unique to activation, only to VB. So, you can get an import code from anyone who has one for Big Al, for example.
EDIT: Also,
here's a resource created by the Vocatwitter community compiling V2 import coddes for all V2 VBs.
You can get the V2 Import Tool from Zero-G
here, or if that doesn't work, feel free to DM me nad I'm happy to share it!