V-brakes are incompatible with your Tiagra brake levers, or any other road levers. If you want to use v-brakes you would need to install a travel agent pulley.
I use v-brakes on my touring bike with v-brake specific drop levers. They require significantly less hand strength compared to my old cantilevers to operate. The cantilevers I used were the very nice Shimano BR-R550s. The v-brakes I now use are some dirt cheap Tektros. V-brakes are simply a fundamentally better design than cantilevers. The reasons so many touring bikes use cantilevers are road lever compatibility, visual appearance, and tradition.
You are correct; however there is a fix as you have mentioned.
My wife wanted V-brakes on her bike after she saw them on another bike in the shop that was building her frame. If a Travel Agent is used, they do work exceptionally well with STI shifters. The Travel Agents have over 17,000 miles on them, and I just recently changed the cables. They never needed adjustment during that time. She is a firm believer in function over form, especially where brakes are concerned.
I run Paul cantis on 2 of my bikes. Coupled with Kool Stop salmon pads they are more than adequate. But, what happens when you put the same brake pads on v-brakes; they work exceptionally well
