So the Keiser computer reads from the spin bike not from the vector pedals?
Sounds like one of your pedals isn’t sending power data so you only get half your power. Can you use your phone app to tell you left right balance?
If this is the case I see people have had this problem for a while (the last comment is useful) Garmin Vector 3 power way too low - #9 by Paul_Yeatman
And Garmin has a suggestion page about it https://support.garmin.com/en-AU/?faq=8XvDuPCDpD6MDaAqTuHyy8
I’ve also read that you need to be careful if you take the battery cap off. When you re tighten them people have stripped the thread easily so be careful and look up what torque it should be.