That's a toughie.
When I was young, we had to walk to school up hill in 3 to 4 foot of snow and the walk back home was also up hill, which made for a very tiring day. Our hair would freeze and I even saw birds frozen in bird baths who couldn't fly away. Poor little things.
I would say if you can stand in -40ºC or F and you still have dexterity in your fingers, you can go that low. The type of vinyl probably won't matter as it will freeze long before your fingers, but with some magic weasel p!ss, you should be able to pull it off.
Seriously, you shouldn't be putting vinyl on anything in much under 45º. Regardless of what these masters say, the adhesive won't take effect for weeks. Either bring your jobs inside [unless these were the temperatures inside your shop] or tell your customers to wait for warmer weather.