Approach vref is selected on the INIT page of the FMC. Depending on the landing flaps cpnfig, speed will vary.

And always set V2 prior to takeoff 

You have a higher stall speed during takeoff, compared to during the approach (difference around 15-20kts),

Vref is the stallspeed x 1.3
whereas
V2 is the SINGLE engine TAKEOFF safety speed, therefore to able to meet all the criterias in the various climb segments, the speed has to be quite low so the aircraft can gain as much height as possible. Still within a good margin from stallspeed though.

But you (hopefully ) have 2 operating engines so you climb like a rocket  8)