ok, so if I understand correctly: if you change the Vpin value to 0 then BLYNK_WRITE is not activated? Im rather new to this all as well, so I don’t see why as the value clearly changes. perhaps its a bug? Anyway have you tried setting the max slider value to 0.1-10 ?
another thing you can try is using BLYNK_WRITE(V1) directly and see if that works.
edit: by the way: thank you! You’ve inadvertently solved one of my problems. Before reading this post I wasn’t aware of blynk_write_default() and as I have 49 pins to manage I had 49 instances of blynk_write().