Viper can go mid, safe lane or solo offlane with this build.

Your ability choice would be in direct response to the kind of lane you have.

Having one in each is vital.
Solo offlane? Max 3rd.
Easy mid aiming for last htis/denies? Max 2nd
Dual lane which you owning? Max 1st.

During a good lane you should be near the melee creeps so you have the ability to use ur first ability on enemies who get too close. In addition if they have no backup you can have a few right clicks aswell!