you can mount vbrakes but as has been said, the frame does not have a cable stop for the housing between frame and brakes. easiest solution is to drill the cable outlet hole a bit larger and run the housing from the vbrake through the top tube to the cable inlet which it then uses as stop. there is NO need to drill the cable inlet larger!
if it is worth it for you to "destroy" the frame only for using vbrakes on the rear is something you need to decide. it certainly looses some "market value" if you do it.
you can mount a cable stop to the seat post clamp bolt which would look like this or nicer if you use a custom made adapter
the frame on the photo has been drilled already so the adapter is kinda useless.
the nicest and most advanced solution i have seen so far is this one
i'd say the roller thingy is custom made and could still be optimized a little, the brake arms are a bit too far apart but that's only a detail.
my solution was to run vbrakes on the front and XTR cantis with koolstop pads on the rear with Avid SpeedDial levers. they work well with Vbrakes and cantis. you could also use a Vbrake lever for the front and a canti lever for the rear but they would look different. either with seperate shifter pods/grip shifters or e.g. XT or XTR STIs which are available with canti and vbrake levers.
good and properly adjusted cantis are more than powerful enough on the rear
Carsten