hi. i tried lot of times with fenix5x plus and 2 phones, but i'm not able to update the pedals to 3.8
i also removed the batteries, but i'm still not able to install the update.
The update should be between the computer (in my case edge 1030) and the vector 3.
I'm not sure why an iPhone vs other smart phone should have impact on that.
Also I tried several times without success. I got a replacement 5 (five) days ago as previous vector 3 was broken. After the 5th update attempt it started to complain the low battery. This would had been the first ride with new Vector 3. I'm sure the battery was not new as one side it was Varta, another side it was Energiser. A replaced to a new one (double one). No more complain, however update still does not work.
I used my iPhone x tp update the pedals and it appeared to simply get stuck. If you are trying to update from your phone, try deleting the Connect app and reinstalling it. Also remove the batteries from both pedals to reset them. After this I found that my Garmin 830 said the update was successful but the iPhone app was showing the pedals stuck at the previous version. Following the procedure I described sorted the problem.