No doubt your VFT's are from tissue culture, but I am no expert in determining how old they are, prior to purchase or what time of year they think it is. This is the time of year for VFT's to be in the process of going dormant and I'm not sure just what yours are wanting to do. When I took this hobby seriously, two years ago, I bought a VFT in June, and a trio in a pot in late August. I grew them open tray, on a window sill and let them naturally get reduced photoperiod and cooler temps, before I put them in the fridge for dormancy. I was rewarded with flowers in the spring. so my plants responded well from their late summer purchase, followed by a dormancy.
Where do you live? If it were me, I would acclimate the plant to outside and let it naturally get a reduced photoperiod and temperature. Once the overnoight lows are forecasted to be lower than 30 (they can take freezes), you could either mulch them up and attempt to winter them over, as some people in the North do, or you can the now asleep plant and put in the refrigerator to finish out its dormancy period.
Not sure why it is yellowing.