You didn’t mention which trainer you’re using with VirtualPower, but I’m here to tell you that VP is just a rough approximation of your power, and one that is very easily affected by other factors like trainer and tire pressure, as well as the relative accuracy of the VP curve in question.
We answer this question under the “VirtualPower Watts vs. Power Meter Watts” section of the following article, and there is an explanation of the factors that affect VP above that:
Just to clear up any confusion, our best recommendation is to re-test your FTP with your new Stages power meter and continue training with your power meter. We always recommend a re-test when changing the method of power measurement, and you’ll likely want to train with the Stages on the trainer since you can also use it outdoors.
I hope this helps! Cheers!