I can remember I had the same issue sometimes when using the Qt SDK together with V-Play SDK. The reason was that the same project setup was used for both Qt SDKs and they sometimes overlapped each other. I solved it, by always having the V-Play SDK Qt Creator open which prepends to overwrite the project settings. In any case, deleting the .pro.user file solved the problem as Alex explained above, so you could also try that.
Is there any reason why you keep the Qt SDK in parallel? The V-Play SDK is a full copy of the Qt SDK and you have the same packages available for the online installer, so you can also develop pure Qt applications without using V-Play with the V-Play SDK!