mxz_chris, You have to change out your underlayer(s) as needed for the conditions and the type of riding you're doing just like you would if you weren't riding with a Tek vest. With mine, I wear a long sleave synthetic shirt and then a mid-weight fleece, the vest and then my shell when the temps were in the teens. But when you move around alot (aggresive riding, off trail or digging out a sled) you sweat up. Open up your pit zips and the back vent and you cool down again.
It's just a matter of trying out combos that'll work for you. REMEMBER only wear synthetics. NO COTTON!