If you have any version of Sonar that came with V-Vocal (which you do if you own 8) then you should be able to just do a custom/advanced install of that old version, only install V-Vocal and then it will show up in Platinum.
Then, hopefully, SPlat will be able to read the clips again.
HOWEVER... if that doesn't work then yeah... install 8.5 (or whatever the project was last modified in), open the project and bounce the V-V clip(s) so you can import them into your SPlat project.
Also maybe take a look in the Audio folder of the project to see if the original/unedited wave is there. I don't know if VV retains the original on active VV edits.
BTW... when doing stuff like this (VV, Melodyne, Audiosnap, VocalSync, etc) best to bounce the work so it doesn't get borked like this as time moves on.